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Re: [PATCH v2] x86: Avoid using .byte for instructions where safe to do so

To: "Andrew Cooper" <andrew.cooper3@xxxxxxxxxx>, Xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>

From: "Teddy Astie" <teddy.astie@xxxxxxxxxx>

Date: Thu, 09 Apr 2026 12:22:04 +0000

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Cc: "Jan Beulich" <jbeulich@xxxxxxxx>, "Roger Pau Monné" <roger.pau@xxxxxxxxxx>

Delivery-date: Thu, 09 Apr 2026 12:22:13 +0000

Feedback-id: 30504962:30504962.20260409:md

List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

Le 09/04/2026 à 13:43, Andrew Cooper a écrit : > The new toolchain baseline knows XGETBV, VPXOR and VPOR. > > For the other cases using .byte, annotate the toolchain minima. > > No functional change. > > Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx> > --- > CC: Jan Beulich <jbeulich@xxxxxxxx> > CC: Roger Pau Monné <roger.pau@xxxxxxxxxx> > CC: Teddy Astie <teddy.astie@xxxxxxxxxx> > > v2 > * CLZERO, {WR,RD}PKRU can't be named yet. > > Pull out of previous series as it's somewhat unrelated. The XSAVE cleanup has > other prerequiesites before it can move away from .byte. > --- > xen/arch/x86/arch.mk | 4 +++ > xen/arch/x86/include/asm/asm-defns.h | 1 + > xen/arch/x86/include/asm/msr.h | 2 ++ > xen/arch/x86/include/asm/prot-key.h | 4 +-- > xen/arch/x86/include/asm/xstate.h | 3 +-- > xen/arch/x86/x86_emulate/0f01.c | 2 +- > xen/arch/x86/x86_emulate/x86_emulate.c | 34 ++++++++++++-------------- > 7 files changed, 27 insertions(+), 23 deletions(-) > > diff --git a/xen/arch/x86/arch.mk b/xen/arch/x86/arch.mk > index 0b42e6312fac..cd0602a79aaf 100644 > --- a/xen/arch/x86/arch.mk > +++ b/xen/arch/x86/arch.mk > @@ -17,7 +17,11 @@ CFLAGS-$(CONFIG_CC_IS_GCC) += -malign-data=abi > $(call cc-options-add,CFLAGS,CC,$(EMBEDDED_EXTRA_CFLAGS)) > $(call cc-option-add,CFLAGS,CC,-Wnested-externs) > $(call as-option-add,CFLAGS,CC,".equ \"x\"$(comma)1",-DHAVE_AS_QUOTED_SYM) > + > +# Binutils >= 2.31, Clang >= 7 > $(call as-option-add,CFLAGS,CC,"movdiri > %rax$(comma)(%rax)",-DHAVE_AS_MOVDIR) > + > +# Binutils >= 2.33, Clang >= 9 > $(call as-option-add,CFLAGS,CC,"enqcmd (%rax)$(comma)%rax",-DHAVE_AS_ENQCMD) > > # Check to see whether the assembler supports the .nop directive. > diff --git a/xen/arch/x86/include/asm/asm-defns.h > b/xen/arch/x86/include/asm/asm-defns.h > index 239dc3af096c..dc9b3ce272fd 100644 > --- a/xen/arch/x86/include/asm/asm-defns.h > +++ b/xen/arch/x86/include/asm/asm-defns.h > @@ -1,5 +1,6 @@ > #include <asm/page-bits.h> > > +/* binutils >= 2.26 or Clang >= 3.8 */ > .macro clzero > .byte 0x0f, 0x01, 0xfc > .endm > diff --git a/xen/arch/x86/include/asm/msr.h b/xen/arch/x86/include/asm/msr.h > index 941a7612f4ba..1377d156f4e1 100644 > --- a/xen/arch/x86/include/asm/msr.h > +++ b/xen/arch/x86/include/asm/msr.h > @@ -63,6 +63,8 @@ static inline void wrmsrns(uint32_t msr, uint64_t val) > /* > * WRMSR is 2 bytes. WRMSRNS is 3 bytes. Pad WRMSR with a redundant CS > * prefix to avoid a trailing NOP. > + * > + * Binutils >= 2.40, Clang >= 16 > */ > alternative_input(".byte 0x2e; wrmsr", > ".byte 0x0f,0x01,0xc6", X86_FEATURE_WRMSRNS, > diff --git a/xen/arch/x86/include/asm/prot-key.h > b/xen/arch/x86/include/asm/prot-key.h > index 8fb15b5c32e9..e8550e0c9203 100644 > --- a/xen/arch/x86/include/asm/prot-key.h > +++ b/xen/arch/x86/include/asm/prot-key.h > @@ -19,7 +19,7 @@ static inline uint32_t rdpkru(void) > { > uint32_t pkru; > > - asm volatile ( ".byte 0x0f,0x01,0xee" > + asm volatile ( ".byte 0x0f,0x01,0xee" /* binutils >= 2.26 or Clang >= > 3.8 */ > : "=a" (pkru) : "c" (0) : "dx" ); > > return pkru; > @@ -27,7 +27,7 @@ static inline uint32_t rdpkru(void) > > static inline void wrpkru(uint32_t pkru) > { > - asm volatile ( ".byte 0x0f,0x01,0xef" > + asm volatile ( ".byte 0x0f,0x01,0xef" /* binutils >= 2.26 or Clang >= > 3.8 */ > :: "a" (pkru), "d" (0), "c" (0) ); > } > > diff --git a/xen/arch/x86/include/asm/xstate.h > b/xen/arch/x86/include/asm/xstate.h > index c96d75e38b25..0519379edb57 100644 > --- a/xen/arch/x86/include/asm/xstate.h > +++ b/xen/arch/x86/include/asm/xstate.h > @@ -118,8 +118,7 @@ static inline uint64_t xgetbv(unsigned int index) > uint32_t lo, hi; > > ASSERT(index); /* get_xcr0() should be used instead. */ > - asm volatile ( ".byte 0x0f,0x01,0xd0" /* xgetbv */ > - : "=a" (lo), "=d" (hi) : "c" (index) ); > + asm volatile ( "xgetbv" : "=a" (lo), "=d" (hi) : "c" (index) ); > > return lo | ((uint64_t)hi << 32); > } > diff --git a/xen/arch/x86/x86_emulate/0f01.c b/xen/arch/x86/x86_emulate/0f01.c > index 4d36c7d289a5..87d338f0c74a 100644 > --- a/xen/arch/x86/x86_emulate/0f01.c > +++ b/xen/arch/x86/x86_emulate/0f01.c > @@ -122,7 +122,7 @@ int x86emul_0f01(struct x86_emulate_state *s, > { > case vex_none: /* serialize */ > host_and_vcpu_must_have(serialize); > - asm volatile ( ".byte 0x0f, 0x01, 0xe8" ); > + asm volatile ( ".byte 0x0f, 0x01, 0xe8" ); /* Binutils >= 2.34, > Clang >= 11 */ > break; > case vex_f2: /* xsusldtrk */ > vcpu_must_have(tsxldtrk); > diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c > b/xen/arch/x86/x86_emulate/x86_emulate.c > index 11d145e17723..e58735ee9590 100644 > --- a/xen/arch/x86/x86_emulate/x86_emulate.c > +++ b/xen/arch/x86/x86_emulate/x86_emulate.c > @@ -4748,27 +4748,25 @@ x86_emulate( > */ > if ( vex.l ) > { > - /* vpxor %xmmN, %xmmN, %xmmN */ > - asm volatile ( ".byte 0xc5,0xf9,0xef,0xc0" ); > - asm volatile ( ".byte 0xc5,0xf1,0xef,0xc9" ); > - asm volatile ( ".byte 0xc5,0xe9,0xef,0xd2" ); > - asm volatile ( ".byte 0xc5,0xe1,0xef,0xdb" ); > - asm volatile ( ".byte 0xc5,0xd9,0xef,0xe4" ); > - asm volatile ( ".byte 0xc5,0xd1,0xef,0xed" ); > - asm volatile ( ".byte 0xc5,0xc9,0xef,0xf6" ); > - asm volatile ( ".byte 0xc5,0xc1,0xef,0xff" ); > + asm volatile ( "vpxor %xmm0, %xmm0, %xmm0" ); > + asm volatile ( "vpxor %xmm1, %xmm1, %xmm1" ); > + asm volatile ( "vpxor %xmm2, %xmm2, %xmm2" ); > + asm volatile ( "vpxor %xmm3, %xmm3, %xmm3" ); > + asm volatile ( "vpxor %xmm4, %xmm4, %xmm4" ); > + asm volatile ( "vpxor %xmm5, %xmm5, %xmm5" ); > + asm volatile ( "vpxor %xmm6, %xmm6, %xmm6" ); > + asm volatile ( "vpxor %xmm7, %xmm7, %xmm7" ); > } > else > { > - /* vpor %xmmN, %xmmN, %xmmN */ > - asm volatile ( ".byte 0xc5,0xf9,0xeb,0xc0" ); > - asm volatile ( ".byte 0xc5,0xf1,0xeb,0xc9" ); > - asm volatile ( ".byte 0xc5,0xe9,0xeb,0xd2" ); > - asm volatile ( ".byte 0xc5,0xe1,0xeb,0xdb" ); > - asm volatile ( ".byte 0xc5,0xd9,0xeb,0xe4" ); > - asm volatile ( ".byte 0xc5,0xd1,0xeb,0xed" ); > - asm volatile ( ".byte 0xc5,0xc9,0xeb,0xf6" ); > - asm volatile ( ".byte 0xc5,0xc1,0xeb,0xff" ); > + asm volatile ( "vpor %xmm0, %xmm0, %xmm0" ); > + asm volatile ( "vpor %xmm1, %xmm1, %xmm1" ); > + asm volatile ( "vpor %xmm2, %xmm2, %xmm2" ); > + asm volatile ( "vpor %xmm3, %xmm3, %xmm3" ); > + asm volatile ( "vpor %xmm4, %xmm4, %xmm4" ); > + asm volatile ( "vpor %xmm5, %xmm5, %xmm5" ); > + asm volatile ( "vpor %xmm6, %xmm6, %xmm6" ); > + asm volatile ( "vpor %xmm7, %xmm7, %xmm7" ); > } > > ASSERT(!state->simd_size); Reviewed-by: Teddy Astie <teddy.astie@xxxxxxxxxx> -- Teddy Astie | Vates XCP-ng Developer XCP-ng & Xen Orchestra - Vates solutions web: https://vates.tech

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