Re: [PATCH v3 2/2] x86/copy_mc: Introduce copy_mc_generic()

[Vivek Goyal <vgoyal@redhat.com>]

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().

[..]
> +/*
> + * 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?

> +	 * 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).
On the other hand copy_mc_fragile() handles MCE recovery only on
source and non-MCE recovery on destination.

Thanks
Vivek