Re: [PATCHV2 3/3] x86, ras: Add mcsafe_memcpy() function to recover from machine checks

From: Dan Williams
Date: Tue Dec 15 2015 - 12:45:12 EST

On Tue, Dec 15, 2015 at 5:11 AM, Borislav Petkov <bp@xxxxxxxxx> wrote:
> On Thu, Dec 10, 2015 at 04:21:50PM -0800, Tony Luck wrote:
>> Using __copy_user_nocache() as inspiration create a memory copy
>> routine for use by kernel code with annotations to allow for
>> recovery from machine checks.
>> Notes:
>> 1) Unlike the original we make no attempt to copy all the bytes
>> up to the faulting address. The original achieves that by
>> re-executing the failing part as a byte-by-byte copy,
>> which will take another page fault. We don't want to have
>> a second machine check!
>> 2) Likewise the return value for the original indicates exactly
>> how many bytes were not copied. Instead we provide the physical
>> address of the fault (thanks to help from do_machine_check()
>> 3) Provide helpful macros to decode the return value.
>> Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx>
>> ---
>> arch/x86/include/asm/uaccess_64.h | 5 +++
>> arch/x86/kernel/x8664_ksyms_64.c | 2 +
>> arch/x86/lib/copy_user_64.S | 91 +++++++++++++++++++++++++++++++++++++++
>> 3 files changed, 98 insertions(+)
> ...
>> + * mcsafe_memcpy - Uncached memory copy with machine check exception handling
>> + * Note that we only catch machine checks when reading the source addresses.
>> + * Writes to target are posted and don't generate machine checks.
>> + * This will force destination/source out of cache for more performance.
> ... and the non-temporal version is the optimal one even though we're
> defaulting to copy_user_enhanced_fast_string for memcpy on modern Intel
> CPUs...?

At least the pmem driver use case does not want caching of the
source-buffer since that is the raw "disk" media. I.e. in
pmem_do_bvec() we'd use this to implement memcpy_from_pmem().
However, caching the destination-buffer may prove beneficial since
that data is likely to be consumed immediately by the thread that
submitted the i/o.
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