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

Kirill A. Shutemov
2025-07-03T14:10:49Z

On Thu, Jul 03, 2025 at 01:15:52PM +0100, David Laight wrote:
> 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.

__inline_memcpy/memset_safe()?

> ...
> > > > 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.

Okay, I see what you are saying.

> > > 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.

The patch below seems to do the trick: compiler optimizes out the
multiplication for v == 0.

It would be nice to avoid it for X86_FEATURE_FSRM, but we cannot use
cpu_feature_enabled() here as <asm/cpufeature.h> depends on
<asm/string.h>.

I cannot say I like the result.

Any suggestions?

diff --git a/arch/x86/include/asm/string.h b/arch/x86/include/asm/string.h
index becb9ee3bc8a..c7644a6f426b 100644
--- a/arch/x86/include/asm/string.h
+++ b/arch/x86/include/asm/string.h
@@ -35,16 +35,27 @@ static __always_inline void *__inline_memcpy(void *to, const void *from, size_t

static __always_inline void *__inline_memset(void *addr, int v, size_t len)
{
+ unsigned long val = v;
void *ret = addr;

+ if (IS_ENABLED(CONFIG_X86_64)) {
+ /*
+ * Fill all bytes by value in byte 0.
+ *
+ * To be used in rep_stos_alternative()i
+ */
+ val &= 0xff;
+ val *= 0x0101010101010101;
+ }
+
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" (v)
- : "memory", _ASM_SI, _ASM_DX);
+ : "a" (val)
+ : "memory");

return ret + len;
}
diff --git a/arch/x86/lib/clear_page_64.S b/arch/x86/lib/clear_page_64.S
index 47b613690f84..3ef7d796deb3 100644
--- a/arch/x86/lib/clear_page_64.S
+++ b/arch/x86/lib/clear_page_64.S
@@ -58,23 +58,15 @@ EXPORT_SYMBOL_GPL(clear_page_erms)
* Default memset.
* Input:
* rdi destination
- * rsi scratch
* rcx count
* 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
-
cmpq $64,%rcx
jae .Lunrolled

--
Kiryl Shutsemau / Kirill A. Shutemov