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

From: Borislav Petkov
Date: Tue Dec 15 2015 - 08:11:46 EST

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

Btw, it should be also inside an ifdef if we're going to ifdef


ECO tip #101: Trim your mails when you reply.
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at