x86, uaccess: introduce copy_from_iter_flushcache for pmem / cache-bypass operations
The pmem driver has a need to transfer data with a persistent memory
destination and be able to rely on the fact that the destination writes are not
cached. It is sufficient for the writes to be flushed to a cpu-store-buffer
(non-temporal / "movnt" in x86 terms), as we expect userspace to call fsync()
to ensure data-writes have reached a power-fail-safe zone in the platform. The
fsync() triggers a REQ_FUA or REQ_FLUSH to the pmem driver which will turn
around and fence previous writes with an "sfence".
Implement a __copy_from_user_inatomic_flushcache, memcpy_page_flushcache, and
memcpy_flushcache, that guarantee that the destination buffer is not dirty in
the cpu cache on completion. The new copy_from_iter_flushcache and sub-routines
will be used to replace the "pmem api" (include/linux/pmem.h +
arch/x86/include/asm/pmem.h). The availability of copy_from_iter_flushcache()
and memcpy_flushcache() are gated by the CONFIG_ARCH_HAS_UACCESS_FLUSHCACHE
config symbol, and fallback to copy_from_iter_nocache() and plain memcpy()
otherwise.
This is meant to satisfy the concern from Linus that if a driver wants to do
something beyond the normal nocache semantics it should be something private to
that driver [1], and Al's concern that anything uaccess related belongs with
the rest of the uaccess code [2].
The first consumer of this interface is a new 'copy_from_iter' dax operation so
that pmem can inject cache maintenance operations without imposing this
overhead on other dax-capable drivers.
[1]: https://lists.01.org/pipermail/linux-nvdimm/2017-January/008364.html
[2]: https://lists.01.org/pipermail/linux-nvdimm/2017-April/009942.html
Cc: <x86@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Jeff Moyer <jmoyer@redhat.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Toshi Kani <toshi.kani@hpe.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Matthew Wilcox <mawilcox@microsoft.com>
Reviewed-by: Ross Zwisler <ross.zwisler@linux.intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
diff --git a/lib/Kconfig b/lib/Kconfig
index 0c8b78a..2d1c4b3 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -548,6 +548,9 @@
config ARCH_HAS_PMEM_API
bool
+config ARCH_HAS_UACCESS_FLUSHCACHE
+ bool
+
config ARCH_HAS_MMIO_FLUSH
bool
diff --git a/lib/iov_iter.c b/lib/iov_iter.c
index f835964..c9a6906 100644
--- a/lib/iov_iter.c
+++ b/lib/iov_iter.c
@@ -615,6 +615,28 @@ size_t copy_from_iter_nocache(void *addr, size_t bytes, struct iov_iter *i)
}
EXPORT_SYMBOL(copy_from_iter_nocache);
+#ifdef CONFIG_ARCH_HAS_UACCESS_FLUSHCACHE
+size_t copy_from_iter_flushcache(void *addr, size_t bytes, struct iov_iter *i)
+{
+ char *to = addr;
+ if (unlikely(i->type & ITER_PIPE)) {
+ WARN_ON(1);
+ return 0;
+ }
+ iterate_and_advance(i, bytes, v,
+ __copy_from_user_flushcache((to += v.iov_len) - v.iov_len,
+ v.iov_base, v.iov_len),
+ memcpy_page_flushcache((to += v.bv_len) - v.bv_len, v.bv_page,
+ v.bv_offset, v.bv_len),
+ memcpy_flushcache((to += v.iov_len) - v.iov_len, v.iov_base,
+ v.iov_len)
+ )
+
+ return bytes;
+}
+EXPORT_SYMBOL_GPL(copy_from_iter_flushcache);
+#endif
+
bool copy_from_iter_full_nocache(void *addr, size_t bytes, struct iov_iter *i)
{
char *to = addr;