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SubjectRe: [PATCH v10 3/4] x86, mce: Add __mcsafe_copy()
> You can save yourself this MOV here in what is, I'm assuming, the
> general likely case where @src is aligned and do:
>
> /* check for bad alignment of source */
> testl $7, %esi
> /* already aligned? */
> jz 102f
>
> movl %esi,%ecx
> subl $8,%ecx
> negl %ecx
> subl %ecx,%edx
> 0: movb (%rsi),%al
> movb %al,(%rdi)
> incq %rsi
> incq %rdi
> decl %ecx
> jnz 0b

The "testl $7, %esi" just checks the low three bits ... it doesn't
change %esi. But the code from the "subl $8" on down assumes that
%ecx is a number in [1..7] as the count of bytes to copy until we
achieve alignment.

So your "movl %esi,%ecx" needs to be somthing that just copies the
low three bits and zeroes the high part of %ecx. Is there a cute
way to do that in x86 assembler?

> Why aren't we pushing %r12-%r15 on the stack after the "jz 17f" above
> and using them too and thus copying a whole cacheline in one go?
>
> We would need to restore them when we're done with the cacheline-wise
> shuffle, of course.

I copied that loop from arch/x86/lib/copy_user_64.S:__copy_user_nocache()
I guess the answer depends on whether you generally copy enough
cache lines to save enough time to cover the cost of saving and
restoring those registers.

-Tony

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