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

David Laight
2025-07-03T12:15:57Z

On Thu, 3 Jul 2025 13:39:57 +0300
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> 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@linux.intel.com> wrote:
> >
> > > Extract memcpy and memset functions from copy_user_generic() and
> > > __clear_user().
> > >
> > > They can be used as inline memcpy and memset instead of the GCC builtins
> > > whenever necessary. LASS requires them to handle text_poke.
> >
> > Except they contain the fault handlers so aren't generic calls.
>
> That's true. I will add a comment to clarify it.

They need renaming.

...
> > > 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.
>
> 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.

> > It is also almost certainly a waste of time.
> > Pretty much all the calls will be for a constant 0x00.
> > Rename it all memzero() ...
>
> text_poke_memset() is not limited to zeroing.

But you don't want the overhead of extending the constant
on all the calls - never mind reserving %rdx to do it.
Maybe define a function that requires the caller to have
done the 'dirty work' - so any code that wants memzero()
just passes zero.
Or do the multiply in the C code where it will get optimised
away for constant zero.
You do get the multiply for the 'rep stosb' case - but that
is always going to be true unless you complicate things further.

David