Re: [PATCH 4.4 26/28] x86, pmem: fix broken __copy_user_nocache cache-bypass assumptions

Dan Williams
2017-05-09T17:11:00Z

On Tue, May 9, 2017 at 10:00 AM, Ben Hutchings
<ben.hutchings@codethink.co.uk> wrote:
> On Tue, 2017-04-25 at 16:08 +0100, Greg Kroah-Hartman wrote:
>> 4.4-stable review patch. If anyone has any objections, please let me know.
>>
>> ------------------
>>
>> From: Dan Williams <dan.j.williams@intel.com>
>>
>> commit 11e63f6d920d6f2dfd3cd421e939a4aec9a58dcd upstream.
> [...]
>> + if (iter_is_iovec(i)) {
>> + unsigned long flushed, dest = (unsigned long) addr;
>> +
>> + if (bytes < 8) {
>> + if (!IS_ALIGNED(dest, 4) || (bytes != 4))
>> + __arch_wb_cache_pmem(addr, 1);
> [...]
>
> What if the write crosses a cache line boundary? I think you need the
> following fix-up (untested, I don't have this kind of hardware).
>
> Ben.
>
> ---
> From: Ben Hutchings <ben.hutchings@codethink.co.uk>
> Subject: x86, pmem: Fix cache flushing for iovec write < 8 bytes
>
> Commit 11e63f6d920d added cache flushing for unaligned writes from an
> iovec, covering the first and last cache line of a >= 8 byte write and
> the first cache line of a < 8 byte write. But an unaligned write of
> 2-7 bytes can still cover two cache lines, so make sure we flush both
> in that case.
>
> Fixes: 11e63f6d920d ("x86, pmem: fix broken __copy_user_nocache ...")
> Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
> ---
> arch/x86/include/asm/pmem.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/arch/x86/include/asm/pmem.h b/arch/x86/include/asm/pmem.h
> index d5a22bac9988..0ff8fe71b255 100644
> --- a/arch/x86/include/asm/pmem.h
> +++ b/arch/x86/include/asm/pmem.h
> @@ -98,7 +98,7 @@ static inline size_t arch_copy_from_iter_pmem(void *addr, size_t bytes,
>
> if (bytes < 8) {
> if (!IS_ALIGNED(dest, 4) || (bytes != 4))
> - arch_wb_cache_pmem(addr, 1);
> + arch_wb_cache_pmem(addr, bytes);

Yes, this looks correct to me. I deeply appreciate your attention to
detail, Ben.