powerpc/mm/radix: Workaround prefetch issue with KVM
There's a somewhat architectural issue with Radix MMU and KVM.
When coming out of a guest with AIL (Alternate Interrupt Location, ie,
MMU enabled), we start executing hypervisor code with the PID register
still containing whatever the guest has been using.
The problem is that the CPU can (and will) then start prefetching or
speculatively load from whatever host context has that same PID (if
any), thus bringing translations for that context into the TLB, which
Linux doesn't know about.
This can cause stale translations and subsequent crashes.
Fixing this in a way that is neither racy nor a huge performance
impact is difficult. We could just make the host invalidations always
use broadcast forms but that would hurt single threaded programs for
example.
We chose to fix it instead by partitioning the PID space between guest
and host. This is possible because today Linux only use 19 out of the
20 bits of PID space, so existing guests will work if we make the host
use the top half of the 20 bits space.
We additionally add support for a property to indicate to Linux the
size of the PID register which will be useful if we eventually have
processors with a larger PID space available.
There is still an issue with malicious guests purposefully setting the
PID register to a value in the hosts PID range. Hopefully future HW
can prevent that, but in the meantime, we handle it with a pair of
kludges:
- On the way out of a guest, before we clear the current VCPU in the
PACA, we check the PID and if it's outside of the permitted range
we flush the TLB for that PID.
- When context switching, if the mm is "new" on that CPU (the
corresponding bit was set for the first time in the mm cpumask), we
check if any sibling thread is in KVM (has a non-NULL VCPU pointer
in the PACA). If that is the case, we also flush the PID for that
CPU (core).
This second part is needed to handle the case where a process is
migrated (or starts a new pthread) on a sibling thread of the CPU
coming out of KVM, as there's a window where stale translations can
exist before we detect it and flush them out.
A future optimization could be added by keeping track of whether the
PID has ever been used and avoid doing that for completely fresh PIDs.
We could similarily mark PIDs that have been the subject of a global
invalidation as "fresh". But for now this will do.
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
[mpe: Rework the asm to build with CONFIG_PPC_RADIX_MMU=n, drop
unneeded include of kvm_book3s_asm.h]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
diff --git a/arch/powerpc/mm/pgtable-radix.c b/arch/powerpc/mm/pgtable-radix.c
index 5cc50d4..671a45d 100644
--- a/arch/powerpc/mm/pgtable-radix.c
+++ b/arch/powerpc/mm/pgtable-radix.c
@@ -25,6 +25,9 @@
#include <trace/events/thp.h>
+unsigned int mmu_pid_bits;
+unsigned int mmu_base_pid;
+
static int native_register_process_table(unsigned long base, unsigned long pg_sz,
unsigned long table_size)
{
@@ -261,11 +264,34 @@ static void __init radix_init_pgtable(void)
for_each_memblock(memory, reg)
WARN_ON(create_physical_mapping(reg->base,
reg->base + reg->size));
+
+ /* Find out how many PID bits are supported */
+ if (cpu_has_feature(CPU_FTR_HVMODE)) {
+ if (!mmu_pid_bits)
+ mmu_pid_bits = 20;
+#ifdef CONFIG_KVM_BOOK3S_HV_POSSIBLE
+ /*
+ * When KVM is possible, we only use the top half of the
+ * PID space to avoid collisions between host and guest PIDs
+ * which can cause problems due to prefetch when exiting the
+ * guest with AIL=3
+ */
+ mmu_base_pid = 1 << (mmu_pid_bits - 1);
+#else
+ mmu_base_pid = 1;
+#endif
+ } else {
+ /* The guest uses the bottom half of the PID space */
+ if (!mmu_pid_bits)
+ mmu_pid_bits = 19;
+ mmu_base_pid = 1;
+ }
+
/*
* Allocate Partition table and process table for the
* host.
*/
- BUILD_BUG_ON_MSG((PRTB_SIZE_SHIFT > 36), "Process table size too large.");
+ BUG_ON(PRTB_SIZE_SHIFT > 36);
process_tb = early_alloc_pgtable(1UL << PRTB_SIZE_SHIFT);
/*
* Fill in the process table.
@@ -339,6 +365,12 @@ static int __init radix_dt_scan_page_sizes(unsigned long node,
if (type == NULL || strcmp(type, "cpu") != 0)
return 0;
+ /* Find MMU PID size */
+ prop = of_get_flat_dt_prop(node, "ibm,mmu-pid-bits", &size);
+ if (prop && size == 4)
+ mmu_pid_bits = be32_to_cpup(prop);
+
+ /* Grab page size encodings */
prop = of_get_flat_dt_prop(node, "ibm,processor-radix-AP-encodings", &size);
if (!prop)
return 0;