Re: [PATCHv8 02/17] x86/asm: Introduce inline memcpy and memset

[Vegard Nossum]

On 03/07/2025 14:15, David Laight wrote:
> On Thu, 3 Jul 2025 13:39:57 +0300
> "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> > On Thu, Jul 03, 2025 at 09:44:17AM +0100, David Laight wrote:
> > > On Tue,  1 Jul 2025 12:58:31 +0300
> > > "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> wrote:
> > > > diff --git a/arch/x86/lib/clear_page_64.S b/arch/x86/lib/clear_page_64.S
> > > > index a508e4a8c66a..47b613690f84 100644
> > > > --- a/arch/x86/lib/clear_page_64.S
> > > > +++ b/arch/x86/lib/clear_page_64.S
> > > > @@ -55,17 +55,26 @@ SYM_FUNC_END(clear_page_erms)
> > > >   EXPORT_SYMBOL_GPL(clear_page_erms)
> > > >   
> > > >   /*
> > > > - * Default clear user-space.
> > > > + * Default memset.
> > > >    * Input:
> > > >    * rdi destination
> > > > + * rsi scratch
> > > >    * rcx count
> > > > - * rax is zero
> > > > + * al is value
> > > >    *
> > > >    * Output:
> > > >    * rcx: uncleared bytes or 0 if successful.
> > > > + * rdx: clobbered
> > > >    */
> > > >   SYM_FUNC_START(rep_stos_alternative)
> > > >   	ANNOTATE_NOENDBR
> > > > +
> > > > +	movzbq %al, %rsi
> > > > +	movabs $0x0101010101010101, %rax
> > > > +
> > > > +	/* RDX:RAX = RAX * RSI */
> > > > +	mulq %rsi
> > >
> > > NAK - you can't do that here.
> > > Neither %rsi nor %rdx can be trashed.
> > > The function has a very explicit calling convention.

That's why we have the clobbers... see below

> > What calling convention? We change the only caller to confirm to this.
>
> The one that is implicit in:
>
> > > > +	asm volatile("1:\n\t"
> > > > +		     ALT_64("rep stosb",
> > > > +			    "call rep_stos_alternative", ALT_NOT(X86_FEATURE_FSRM))
> > > > +		     "2:\n\t"
> > > > +		     _ASM_EXTABLE_UA(1b, 2b)
> > > > +		     : "+c" (len), "+D" (addr), ASM_CALL_CONSTRAINT
> > > > +		     : "a" ((uint8_t)v)
>
> The called function is only allowed to change the registers that
> 'rep stosb' uses - except it can access (but not change)
> all of %rax - not just %al.
>
> See: https://godbolt.org/z/3fnrT3x9r
> In particular note that 'do_mset' must not change %rax.
>
> This is very specific and is done so that the compiler can use
> all the registers.

I feel like you trimmed off the clobbers from the asm() in the context
above. For reference, it is:

+		     : "memory", _ASM_SI, _ASM_DX);

I'm not saying this can't be optimized, but that doesn't seem to be your
complaint -- you say "the called function is only allowed to change
...", but this is not true when we have the clobbers, right?

This is exactly what I fixed with my v7 fixlet to this patch:

https://lore.kernel.org/all/1b96b0ca-5c14-4271-86c1-c305bf052b16@xxxxxxxxxx/


Vegard