7:57 a.m.
On Wed, May 20, 2020 at 12:13 PM Vivek Goyal <vgoyal(a)redhat.com> wrote:
On Tue, May 19, 2020 at 03:12:42PM -0700, Dan Williams wrote:
> The original copy_mc_fragile() implementation had negative performance
> implications since it did not use the fast-string instruction sequence
> to perform copies. For this reason copy_mc_to_kernel() fell back to
> plain memcpy() to preserve performance on platform that did not indicate
> the capability to recover from machine check exceptions. However, that
> capability detection was not architectural and now that some platforms
> can recover from fast-string consumption of memory errors the memcpy()
> fallback now causes these more capable platforms to fail.
>
> Introduce copy_mc_generic() as the fast default implementation of
> copy_mc_to_kernel() and finalize the transition of copy_mc_fragile() to
> be a platform quirk to indicate 'fragility'. With this in place
> copy_mc_to_kernel() is fast and recovery-ready by default regardless of
> hardware capability.
>
> Thanks to Vivek for identifying that copy_user_generic() is not suitable
> as the copy_mc_to_user() backend since the #MC handler explicitly checks
> ex_has_fault_handler().
/me is curious to know why #MC handler mandates use of _ASM_EXTABLE_FAULT().
Even though we could try to handle all faults / exceptions
generically, I think it makes sense to enforce type safety here if
only to support architectures that can only satisfy the minimum
contract of copy_mc_to_user(). For example, if there was some
destination exception other than #PF the contract implied by
copy_mc_to_user() is that exception is not intended to be permissible
in this path. See:
00c42373d397 x86-64: add warning for non-canonical user access address
dereferences
75045f77f7a7 x86/extable: Introduce _ASM_EXTABLE_UA for uaccess fixups
...for examples of other justification for being explicit in these paths.
[..]
> +/*
> + * copy_mc_generic - memory copy with exception handling
> + *
> + * Fast string copy + fault / exception handling. If the CPU does
> + * support machine check exception recovery, but does not support
> + * recovering from fast-string exceptions then this CPU needs to be
> + * added to the copy_mc_fragile_key set of quirks. Otherwise, absent any
> + * machine check recovery support this version should be no slower than
> + * standard memcpy.
> + */
> +SYM_FUNC_START(copy_mc_generic)
> + ALTERNATIVE "jmp copy_mc_fragile", "", X86_FEATURE_ERMS
> + movq %rdi, %rax
> + movq %rdx, %rcx
> +.L_copy:
> + rep movsb
> + /* Copy successful. Return zero */
> + xorl %eax, %eax
> + ret
> +SYM_FUNC_END(copy_mc_generic)
> +EXPORT_SYMBOL_GPL(copy_mc_generic)
> +
> + .section .fixup, "ax"
> +.E_copy:
> + /*
> + * On fault %rcx is updated such that the copy instruction could
> + * optionally be restarted at the fault position, i.e. it
> + * contains 'bytes remaining'. A non-zero return indicates error
> + * to copy_safe() users, or indicate short transfers to
copy_safe() is vestige of terminology of previous patches?
Thanks, yes, I missed this one.
> + * user-copy routines.
> + */
> + movq %rcx, %rax
> + ret
> +
> + .previous
> +
> + _ASM_EXTABLE_FAULT(.L_copy, .E_copy)
A question for my education purposes.
So copy_mc_generic() can handle MCE both on source and destination
addresses? (Assuming some device can generate MCE on stores too).
There's no such thing as #MC on write. #MC is only signaled on consumed poison.
In this case what is specifically being handled is #MC with RIP
pointing at a movq instruction. The fault handler actually does not
know anything about source or destination, it just knows fault /
exception type and the register state.
On the other hand copy_mc_fragile() handles MCE recovery only on
source and non-MCE recovery on destination.
No, there's no difference in capability. #MC can only be raised on a
poison-read in both cases.