arch/mips/kvm/entry.c

/*
 * This file is subject to the terms and conditions of the GNU General Public
 * License.  See the file "COPYING" in the main directory of this archive
 * for more details.
 *
 * Generation of main entry point for the guest, exception handling.
 *
 * Copyright (C) 2012  MIPS Technologies, Inc.
 * Authors: Sanjay Lal <sanjayl@kymasys.com>
 *
 * Copyright (C) 2016 Imagination Technologies Ltd.
 */

#include <linux/kvm_host.h>
#include <asm/msa.h>
#include <asm/setup.h>
#include <asm/uasm.h>

/* Register names */
#define ZERO		0
#define AT		1
#define V0		2
#define V1		3
#define A0		4
#define A1		5

#if _MIPS_SIM == _MIPS_SIM_ABI32
#define T0		8
#define T1		9
#define T2		10
#define T3		11
#endif /* _MIPS_SIM == _MIPS_SIM_ABI32 */

#if _MIPS_SIM == _MIPS_SIM_ABI64 || _MIPS_SIM == _MIPS_SIM_NABI32
#define T0		12
#define T1		13
#define T2		14
#define T3		15
#endif /* _MIPS_SIM == _MIPS_SIM_ABI64 || _MIPS_SIM == _MIPS_SIM_NABI32 */

#define S0		16
#define S1		17
#define T9		25
#define K0		26
#define K1		27
#define GP		28
#define SP		29
#define RA		31

/* Some CP0 registers */
#define C0_HWRENA	7, 0
#define C0_BADVADDR	8, 0
#define C0_ENTRYHI	10, 0
#define C0_STATUS	12, 0
#define C0_CAUSE	13, 0
#define C0_EPC		14, 0
#define C0_EBASE	15, 1
#define C0_CONFIG5	16, 5
#define C0_DDATA_LO	28, 3
#define C0_ERROREPC	30, 0

#define CALLFRAME_SIZ   32

static unsigned int scratch_vcpu[2] = { C0_DDATA_LO };
static unsigned int scratch_tmp[2] = { C0_ERROREPC };

enum label_id {
	label_fpu_1 = 1,
	label_msa_1,
	label_return_to_host,
	label_kernel_asid,
	label_exit_common,
};

UASM_L_LA(_fpu_1)
UASM_L_LA(_msa_1)
UASM_L_LA(_return_to_host)
UASM_L_LA(_kernel_asid)
UASM_L_LA(_exit_common)

static void *kvm_mips_build_enter_guest(void *addr);
static void *kvm_mips_build_ret_from_exit(void *addr);
static void *kvm_mips_build_ret_to_guest(void *addr);
static void *kvm_mips_build_ret_to_host(void *addr);

/**
 * kvm_mips_entry_setup() - Perform global setup for entry code.
 *
 * Perform global setup for entry code, such as choosing a scratch register.
 *
 * Returns:	0 on success.
 *		-errno on failure.
 */
int kvm_mips_entry_setup(void)
{
	/*
	 * We prefer to use KScratchN registers if they are available over the
	 * defaults above, which may not work on all cores.
	 */
	unsigned int kscratch_mask = cpu_data[0].kscratch_mask & 0xfc;

	/* Pick a scratch register for storing VCPU */
	if (kscratch_mask) {
		scratch_vcpu[0] = 31;
		scratch_vcpu[1] = ffs(kscratch_mask) - 1;
		kscratch_mask &= ~BIT(scratch_vcpu[1]);
	}

	/* Pick a scratch register to use as a temp for saving state */
	if (kscratch_mask) {
		scratch_tmp[0] = 31;
		scratch_tmp[1] = ffs(kscratch_mask) - 1;
		kscratch_mask &= ~BIT(scratch_tmp[1]);
	}

	return 0;
}

static void kvm_mips_build_save_scratch(u32 **p, unsigned int tmp,
					unsigned int frame)
{
	/* Save the VCPU scratch register value in cp0_epc of the stack frame */
	UASM_i_MFC0(p, tmp, scratch_vcpu[0], scratch_vcpu[1]);
	UASM_i_SW(p, tmp, offsetof(struct pt_regs, cp0_epc), frame);

	/* Save the temp scratch register value in cp0_cause of stack frame */
	if (scratch_tmp[0] == 31) {
		UASM_i_MFC0(p, tmp, scratch_tmp[0], scratch_tmp[1]);
		UASM_i_SW(p, tmp, offsetof(struct pt_regs, cp0_cause), frame);
	}
}

static void kvm_mips_build_restore_scratch(u32 **p, unsigned int tmp,
					   unsigned int frame)
{
	/*
	 * Restore host scratch register values saved by
	 * kvm_mips_build_save_scratch().
	 */
	UASM_i_LW(p, tmp, offsetof(struct pt_regs, cp0_epc), frame);
	UASM_i_MTC0(p, tmp, scratch_vcpu[0], scratch_vcpu[1]);

	if (scratch_tmp[0] == 31) {
		UASM_i_LW(p, tmp, offsetof(struct pt_regs, cp0_cause), frame);
		UASM_i_MTC0(p, tmp, scratch_tmp[0], scratch_tmp[1]);
	}
}

/**
 * kvm_mips_build_vcpu_run() - Assemble function to start running a guest VCPU.
 * @addr:	Address to start writing code.
 *
 * Assemble the start of the vcpu_run function to run a guest VCPU. The function
 * conforms to the following prototype:
 *
 * int vcpu_run(struct kvm_run *run, struct kvm_vcpu *vcpu);
 *
 * The exit from the guest and return to the caller is handled by the code
 * generated by kvm_mips_build_ret_to_host().
 *
 * Returns:	Next address after end of written function.
 */
void *kvm_mips_build_vcpu_run(void *addr)
{
	u32 *p = addr;
	unsigned int i;

	/*
	 * A0: run
	 * A1: vcpu
	 */

	/* k0/k1 not being used in host kernel context */
	UASM_i_ADDIU(&p, K1, SP, -(int)sizeof(struct pt_regs));
	for (i = 16; i < 32; ++i) {
		if (i == 24)
			i = 28;
		UASM_i_SW(&p, i, offsetof(struct pt_regs, regs[i]), K1);
	}

	/* Save host status */
	uasm_i_mfc0(&p, V0, C0_STATUS);
	UASM_i_SW(&p, V0, offsetof(struct pt_regs, cp0_status), K1);

	/* Save scratch registers, will be used to store pointer to vcpu etc */
	kvm_mips_build_save_scratch(&p, V1, K1);

	/* VCPU scratch register has pointer to vcpu */
	UASM_i_MTC0(&p, A1, scratch_vcpu[0], scratch_vcpu[1]);

	/* Offset into vcpu->arch */
	UASM_i_ADDIU(&p, K1, A1, offsetof(struct kvm_vcpu, arch));

	/*
	 * Save the host stack to VCPU, used for exception processing
	 * when we exit from the Guest
	 */
	UASM_i_SW(&p, SP, offsetof(struct kvm_vcpu_arch, host_stack), K1);

	/* Save the kernel gp as well */
	UASM_i_SW(&p, GP, offsetof(struct kvm_vcpu_arch, host_gp), K1);

	/*
	 * Setup status register for running the guest in UM, interrupts
	 * are disabled
	 */
	UASM_i_LA(&p, K0, ST0_EXL | KSU_USER | ST0_BEV);
	uasm_i_mtc0(&p, K0, C0_STATUS);
	uasm_i_ehb(&p);

	/* load up the new EBASE */
	UASM_i_LW(&p, K0, offsetof(struct kvm_vcpu_arch, guest_ebase), K1);
	uasm_i_mtc0(&p, K0, C0_EBASE);

	/*
	 * Now that the new EBASE has been loaded, unset BEV, set
	 * interrupt mask as it was but make sure that timer interrupts
	 * are enabled
	 */
	uasm_i_addiu(&p, K0, ZERO, ST0_EXL | KSU_USER | ST0_IE);
	uasm_i_andi(&p, V0, V0, ST0_IM);
	uasm_i_or(&p, K0, K0, V0);
	uasm_i_mtc0(&p, K0, C0_STATUS);
	uasm_i_ehb(&p);

	p = kvm_mips_build_enter_guest(p);

	return p;
}

/**
 * kvm_mips_build_enter_guest() - Assemble code to resume guest execution.
 * @addr:	Address to start writing code.
 *
 * Assemble the code to resume guest execution. This code is common between the
 * initial entry into the guest from the host, and returning from the exit
 * handler back to the guest.
 *
 * Returns:	Next address after end of written function.
 */
static void *kvm_mips_build_enter_guest(void *addr)
{
	u32 *p = addr;
	unsigned int i;
	struct uasm_label labels[2];
	struct uasm_reloc relocs[2];
	struct uasm_label *l = labels;
	struct uasm_reloc *r = relocs;

	memset(labels, 0, sizeof(labels));
	memset(relocs, 0, sizeof(relocs));

	/* Set Guest EPC */
	UASM_i_LW(&p, T0, offsetof(struct kvm_vcpu_arch, pc), K1);
	UASM_i_MTC0(&p, T0, C0_EPC);

	/* Set the ASID for the Guest Kernel */
	UASM_i_LW(&p, T0, offsetof(struct kvm_vcpu_arch, cop0), K1);
	UASM_i_LW(&p, T0, offsetof(struct mips_coproc, reg[MIPS_CP0_STATUS][0]),
		  T0);
	uasm_i_andi(&p, T0, T0, KSU_USER | ST0_ERL | ST0_EXL);
	uasm_i_xori(&p, T0, T0, KSU_USER);
	uasm_il_bnez(&p, &r, T0, label_kernel_asid);
	 UASM_i_ADDIU(&p, T1, K1,
		      offsetof(struct kvm_vcpu_arch, guest_kernel_asid));
	/* else user */
	UASM_i_ADDIU(&p, T1, K1,
		     offsetof(struct kvm_vcpu_arch, guest_user_asid));
	uasm_l_kernel_asid(&l, p);

	/* t1: contains the base of the ASID array, need to get the cpu id  */
	/* smp_processor_id */
	uasm_i_lw(&p, T2, offsetof(struct thread_info, cpu), GP);
	/* x4 */
	uasm_i_sll(&p, T2, T2, 2);
	UASM_i_ADDU(&p, T3, T1, T2);
	uasm_i_lw(&p, K0, 0, T3);
#ifdef CONFIG_MIPS_ASID_BITS_VARIABLE
	/* x sizeof(struct cpuinfo_mips)/4 */
	uasm_i_addiu(&p, T3, ZERO, sizeof(struct cpuinfo_mips)/4);
	uasm_i_mul(&p, T2, T2, T3);

	UASM_i_LA_mostly(&p, AT, (long)&cpu_data[0].asid_mask);
	UASM_i_ADDU(&p, AT, AT, T2);
	UASM_i_LW(&p, T2, uasm_rel_lo((long)&cpu_data[0].asid_mask), AT);
	uasm_i_and(&p, K0, K0, T2);
#else
	uasm_i_andi(&p, K0, K0, MIPS_ENTRYHI_ASID);
#endif
	uasm_i_mtc0(&p, K0, C0_ENTRYHI);
	uasm_i_ehb(&p);

	/* Disable RDHWR access */
	uasm_i_mtc0(&p, ZERO, C0_HWRENA);

	/* load the guest context from VCPU and return */
	for (i = 1; i < 32; ++i) {
		/* Guest k0/k1 loaded later */
		if (i == K0 || i == K1)
			continue;
		UASM_i_LW(&p, i, offsetof(struct kvm_vcpu_arch, gprs[i]), K1);
	}

#ifndef CONFIG_CPU_MIPSR6
	/* Restore hi/lo */
	UASM_i_LW(&p, K0, offsetof(struct kvm_vcpu_arch, hi), K1);
	uasm_i_mthi(&p, K0);

	UASM_i_LW(&p, K0, offsetof(struct kvm_vcpu_arch, lo), K1);
	uasm_i_mtlo(&p, K0);
#endif

	/* Restore the guest's k0/k1 registers */
	UASM_i_LW(&p, K0, offsetof(struct kvm_vcpu_arch, gprs[K0]), K1);
	UASM_i_LW(&p, K1, offsetof(struct kvm_vcpu_arch, gprs[K1]), K1);

	/* Jump to guest */
	uasm_i_eret(&p);

	uasm_resolve_relocs(relocs, labels);

	return p;
}

/**
 * kvm_mips_build_exception() - Assemble first level guest exception handler.
 * @addr:	Address to start writing code.
 * @handler:	Address of common handler (within range of @addr).
 *
 * Assemble exception vector code for guest execution. The generated vector will
 * branch to the common exception handler generated by kvm_mips_build_exit().
 *
 * Returns:	Next address after end of written function.
 */
void *kvm_mips_build_exception(void *addr, void *handler)
{
	u32 *p = addr;
	struct uasm_label labels[2];
	struct uasm_reloc relocs[2];
	struct uasm_label *l = labels;
	struct uasm_reloc *r = relocs;

	memset(labels, 0, sizeof(labels));
	memset(relocs, 0, sizeof(relocs));

	/* Save guest k1 into scratch register */
	UASM_i_MTC0(&p, K1, scratch_tmp[0], scratch_tmp[1]);

	/* Get the VCPU pointer from the VCPU scratch register */
	UASM_i_MFC0(&p, K1, scratch_vcpu[0], scratch_vcpu[1]);
	UASM_i_ADDIU(&p, K1, K1, offsetof(struct kvm_vcpu, arch));

	/* Save guest k0 into VCPU structure */
	UASM_i_SW(&p, K0, offsetof(struct kvm_vcpu_arch, gprs[K0]), K1);

	/* Branch to the common handler */
	uasm_il_b(&p, &r, label_exit_common);
	 uasm_i_nop(&p);

	uasm_l_exit_common(&l, handler);
	uasm_resolve_relocs(relocs, labels);

	return p;
}

/**
 * kvm_mips_build_exit() - Assemble common guest exit handler.
 * @addr:	Address to start writing code.
 *
 * Assemble the generic guest exit handling code. This is called by the
 * exception vectors (generated by kvm_mips_build_exception()), and calls
 * kvm_mips_handle_exit(), then either resumes the guest or returns to the host
 * depending on the return value.
 *
 * Returns:	Next address after end of written function.
 */
void *kvm_mips_build_exit(void *addr)
{
	u32 *p = addr;
	unsigned int i;
	struct uasm_label labels[3];
	struct uasm_reloc relocs[3];
	struct uasm_label *l = labels;
	struct uasm_reloc *r = relocs;

	memset(labels, 0, sizeof(labels));
	memset(relocs, 0, sizeof(relocs));

	/*
	 * Generic Guest exception handler. We end up here when the guest
	 * does something that causes a trap to kernel mode.
	 *
	 * Both k0/k1 registers will have already been saved (k0 into the vcpu
	 * structure, and k1 into the scratch_tmp register).
	 *
	 * The k1 register will already contain the kvm_vcpu_arch pointer.
	 */

	/* Start saving Guest context to VCPU */
	for (i = 0; i < 32; ++i) {
		/* Guest k0/k1 saved later */
		if (i == K0 || i == K1)
			continue;
		UASM_i_SW(&p, i, offsetof(struct kvm_vcpu_arch, gprs[i]), K1);
	}

#ifndef CONFIG_CPU_MIPSR6
	/* We need to save hi/lo and restore them on the way out */
	uasm_i_mfhi(&p, T0);
	UASM_i_SW(&p, T0, offsetof(struct kvm_vcpu_arch, hi), K1);

	uasm_i_mflo(&p, T0);
	UASM_i_SW(&p, T0, offsetof(struct kvm_vcpu_arch, lo), K1);
#endif

	/* Finally save guest k1 to VCPU */
	uasm_i_ehb(&p);
	UASM_i_MFC0(&p, T0, scratch_tmp[0], scratch_tmp[1]);
	UASM_i_SW(&p, T0, offsetof(struct kvm_vcpu_arch, gprs[K1]), K1);

	/* Now that context has been saved, we can use other registers */

	/* Restore vcpu */
	UASM_i_MFC0(&p, A1, scratch_vcpu[0], scratch_vcpu[1]);
	uasm_i_move(&p, S1, A1);

	/* Restore run (vcpu->run) */
	UASM_i_LW(&p, A0, offsetof(struct kvm_vcpu, run), A1);
	/* Save pointer to run in s0, will be saved by the compiler */
	uasm_i_move(&p, S0, A0);

	/*
	 * Save Host level EPC, BadVaddr and Cause to VCPU, useful to process
	 * the exception
	 */
	UASM_i_MFC0(&p, K0, C0_EPC);
	UASM_i_SW(&p, K0, offsetof(struct kvm_vcpu_arch, pc), K1);

	UASM_i_MFC0(&p, K0, C0_BADVADDR);
	UASM_i_SW(&p, K0, offsetof(struct kvm_vcpu_arch, host_cp0_badvaddr),
		  K1);

	uasm_i_mfc0(&p, K0, C0_CAUSE);
	uasm_i_sw(&p, K0, offsetof(struct kvm_vcpu_arch, host_cp0_cause), K1);

	/* Now restore the host state just enough to run the handlers */

	/* Switch EBASE to the one used by Linux */
	/* load up the host EBASE */
	uasm_i_mfc0(&p, V0, C0_STATUS);

	uasm_i_lui(&p, AT, ST0_BEV >> 16);
	uasm_i_or(&p, K0, V0, AT);

	uasm_i_mtc0(&p, K0, C0_STATUS);
	uasm_i_ehb(&p);

	UASM_i_LA_mostly(&p, K0, (long)&ebase);
	UASM_i_LW(&p, K0, uasm_rel_lo((long)&ebase), K0);
	uasm_i_mtc0(&p, K0, C0_EBASE);

	if (raw_cpu_has_fpu) {
		/*
		 * If FPU is enabled, save FCR31 and clear it so that later
		 * ctc1's don't trigger FPE for pending exceptions.
		 */
		uasm_i_lui(&p, AT, ST0_CU1 >> 16);
		uasm_i_and(&p, V1, V0, AT);
		uasm_il_beqz(&p, &r, V1, label_fpu_1);
		 uasm_i_nop(&p);
		uasm_i_cfc1(&p, T0, 31);
		uasm_i_sw(&p, T0, offsetof(struct kvm_vcpu_arch, fpu.fcr31),
			  K1);
		uasm_i_ctc1(&p, ZERO, 31);
		uasm_l_fpu_1(&l, p);
	}

	if (cpu_has_msa) {
		/*
		 * If MSA is enabled, save MSACSR and clear it so that later
		 * instructions don't trigger MSAFPE for pending exceptions.
		 */
		uasm_i_mfc0(&p, T0, C0_CONFIG5);
		uasm_i_ext(&p, T0, T0, 27, 1); /* MIPS_CONF5_MSAEN */
		uasm_il_beqz(&p, &r, T0, label_msa_1);
		 uasm_i_nop(&p);
		uasm_i_cfcmsa(&p, T0, MSA_CSR);
		uasm_i_sw(&p, T0, offsetof(struct kvm_vcpu_arch, fpu.msacsr),
			  K1);
		uasm_i_ctcmsa(&p, MSA_CSR, ZERO);
		uasm_l_msa_1(&l, p);
	}

	/* Now that the new EBASE has been loaded, unset BEV and KSU_USER */
	uasm_i_addiu(&p, AT, ZERO, ~(ST0_EXL | KSU_USER | ST0_IE));
	uasm_i_and(&p, V0, V0, AT);
	uasm_i_lui(&p, AT, ST0_CU0 >> 16);
	uasm_i_or(&p, V0, V0, AT);
	uasm_i_mtc0(&p, V0, C0_STATUS);
	uasm_i_ehb(&p);

	/* Load up host GP */
	UASM_i_LW(&p, GP, offsetof(struct kvm_vcpu_arch, host_gp), K1);

	/* Need a stack before we can jump to "C" */
	UASM_i_LW(&p, SP, offsetof(struct kvm_vcpu_arch, host_stack), K1);

	/* Saved host state */
	UASM_i_ADDIU(&p, SP, SP, -(int)sizeof(struct pt_regs));

	/*
	 * XXXKYMA do we need to load the host ASID, maybe not because the
	 * kernel entries are marked GLOBAL, need to verify
	 */

	/* Restore host scratch registers, as we'll have clobbered them */
	kvm_mips_build_restore_scratch(&p, K0, SP);

	/* Restore RDHWR access */
	UASM_i_LA_mostly(&p, K0, (long)&hwrena);
	uasm_i_lw(&p, K0, uasm_rel_lo((long)&hwrena), K0);
	uasm_i_mtc0(&p, K0, C0_HWRENA);

	/* Jump to handler */
	/*
	 * XXXKYMA: not sure if this is safe, how large is the stack??
	 * Now jump to the kvm_mips_handle_exit() to see if we can deal
	 * with this in the kernel
	 */
	UASM_i_LA(&p, T9, (unsigned long)kvm_mips_handle_exit);
	uasm_i_jalr(&p, RA, T9);
	 UASM_i_ADDIU(&p, SP, SP, -CALLFRAME_SIZ);

	uasm_resolve_relocs(relocs, labels);

	p = kvm_mips_build_ret_from_exit(p);

	return p;
}

/**
 * kvm_mips_build_ret_from_exit() - Assemble guest exit return handler.
 * @addr:	Address to start writing code.
 *
 * Assemble the code to handle the return from kvm_mips_handle_exit(), either
 * resuming the guest or returning to the host depending on the return value.
 *
 * Returns:	Next address after end of written function.
 */
static void *kvm_mips_build_ret_from_exit(void *addr)
{
	u32 *p = addr;
	struct uasm_label labels[2];
	struct uasm_reloc relocs[2];
	struct uasm_label *l = labels;
	struct uasm_reloc *r = relocs;

	memset(labels, 0, sizeof(labels));
	memset(relocs, 0, sizeof(relocs));

	/* Return from handler Make sure interrupts are disabled */
	uasm_i_di(&p, ZERO);
	uasm_i_ehb(&p);

	/*
	 * XXXKYMA: k0/k1 could have been blown away if we processed
	 * an exception while we were handling the exception from the
	 * guest, reload k1
	 */

	uasm_i_move(&p, K1, S1);
	UASM_i_ADDIU(&p, K1, K1, offsetof(struct kvm_vcpu, arch));

	/*
	 * Check return value, should tell us if we are returning to the
	 * host (handle I/O etc)or resuming the guest
	 */
	uasm_i_andi(&p, T0, V0, RESUME_HOST);
	uasm_il_bnez(&p, &r, T0, label_return_to_host);
	 uasm_i_nop(&p);

	p = kvm_mips_build_ret_to_guest(p);

	uasm_l_return_to_host(&l, p);
	p = kvm_mips_build_ret_to_host(p);

	uasm_resolve_relocs(relocs, labels);

	return p;
}

/**
 * kvm_mips_build_ret_to_guest() - Assemble code to return to the guest.
 * @addr:	Address to start writing code.
 *
 * Assemble the code to handle return from the guest exit handler
 * (kvm_mips_handle_exit()) back to the guest.
 *
 * Returns:	Next address after end of written function.
 */
static void *kvm_mips_build_ret_to_guest(void *addr)
{
	u32 *p = addr;

	/* Put the saved pointer to vcpu (s1) back into the scratch register */
	UASM_i_MTC0(&p, S1, scratch_vcpu[0], scratch_vcpu[1]);

	/* Load up the Guest EBASE to minimize the window where BEV is set */
	UASM_i_LW(&p, T0, offsetof(struct kvm_vcpu_arch, guest_ebase), K1);

	/* Switch EBASE back to the one used by KVM */
	uasm_i_mfc0(&p, V1, C0_STATUS);
	uasm_i_lui(&p, AT, ST0_BEV >> 16);
	uasm_i_or(&p, K0, V1, AT);
	uasm_i_mtc0(&p, K0, C0_STATUS);
	uasm_i_ehb(&p);
	uasm_i_mtc0(&p, T0, C0_EBASE);

	/* Setup status register for running guest in UM */
	uasm_i_ori(&p, V1, V1, ST0_EXL | KSU_USER | ST0_IE);
	UASM_i_LA(&p, AT, ~(ST0_CU0 | ST0_MX));
	uasm_i_and(&p, V1, V1, AT);
	uasm_i_mtc0(&p, V1, C0_STATUS);
	uasm_i_ehb(&p);

	p = kvm_mips_build_enter_guest(p);

	return p;
}

/**
 * kvm_mips_build_ret_to_host() - Assemble code to return to the host.
 * @addr:	Address to start writing code.
 *
 * Assemble the code to handle return from the guest exit handler
 * (kvm_mips_handle_exit()) back to the host, i.e. to the caller of the vcpu_run
 * function generated by kvm_mips_build_vcpu_run().
 *
 * Returns:	Next address after end of written function.
 */
static void *kvm_mips_build_ret_to_host(void *addr)
{
	u32 *p = addr;
	unsigned int i;

	/* EBASE is already pointing to Linux */
	UASM_i_LW(&p, K1, offsetof(struct kvm_vcpu_arch, host_stack), K1);
	UASM_i_ADDIU(&p, K1, K1, -(int)sizeof(struct pt_regs));

	/*
	 * r2/v0 is the return code, shift it down by 2 (arithmetic)
	 * to recover the err code
	 */
	uasm_i_sra(&p, K0, V0, 2);
	uasm_i_move(&p, V0, K0);

	/* Load context saved on the host stack */
	for (i = 16; i < 31; ++i) {
		if (i == 24)
			i = 28;
		UASM_i_LW(&p, i, offsetof(struct pt_regs, regs[i]), K1);
	}

	/* Restore RDHWR access */
	UASM_i_LA_mostly(&p, K0, (long)&hwrena);
	uasm_i_lw(&p, K0, uasm_rel_lo((long)&hwrena), K0);
	uasm_i_mtc0(&p, K0, C0_HWRENA);

	/* Restore RA, which is the address we will return to */
	UASM_i_LW(&p, RA, offsetof(struct pt_regs, regs[RA]), K1);
	uasm_i_jr(&p, RA);
	 uasm_i_nop(&p);

	return p;
}