On Thu, Aug 07, 2025, Uros Bizjak wrote: > Use memory operand in CMP instruction to avoid usage of a > temporary register. Use %eax register to hold VMX_spec_ctrl > and use it directly in the follow-up WRMSR. > > The new code saves a few bytes by removing two MOV insns, from: > > 2d: 48 8b 7c 24 10 mov 0x10(%rsp),%rdi > 32: 8b bf 48 18 00 00 mov 0x1848(%rdi),%edi > 38: 65 8b 35 00 00 00 00 mov %gs:0x0(%rip),%esi > 3f: 39 fe cmp %edi,%esi > 41: 74 0b je 4e <...> > 43: b9 48 00 00 00 mov $0x48,%ecx > 48: 31 d2 xor %edx,%edx > 4a: 89 f8 mov %edi,%eax > 4c: 0f 30 wrmsr > > to: > > 2d: 48 8b 7c 24 10 mov 0x10(%rsp),%rdi > 32: 8b 87 48 18 00 00 mov 0x1848(%rdi),%eax > 38: 65 3b 05 00 00 00 00 cmp %gs:0x0(%rip),%eax > 3f: 74 09 je 4a <...> > 41: b9 48 00 00 00 mov $0x48,%ecx > 46: 31 d2 xor %edx,%edx > 48: 0f 30 wrmsr > > No functional change intended. > > Signed-off-by: Uros Bizjak <ubizjak@xxxxxxxxx> > Cc: Sean Christopherson <seanjc@xxxxxxxxxx> > Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx> > Cc: Ingo Molnar <mingo@xxxxxxxxxx> > Cc: Borislav Petkov <bp@xxxxxxxxx> > Cc: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> > Cc: "H. Peter Anvin" <hpa@xxxxxxxxx> > --- > arch/x86/kvm/vmx/vmenter.S | 6 ++---- > 1 file changed, 2 insertions(+), 4 deletions(-) > > diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S > index 0a6cf5bff2aa..c65de5de92ab 100644 > --- a/arch/x86/kvm/vmx/vmenter.S > +++ b/arch/x86/kvm/vmx/vmenter.S > @@ -118,13 +118,11 @@ SYM_FUNC_START(__vmx_vcpu_run) > * and vmentry. > */ > mov 2*WORD_SIZE(%_ASM_SP), %_ASM_DI > - movl VMX_spec_ctrl(%_ASM_DI), %edi > - movl PER_CPU_VAR(x86_spec_ctrl_current), %esi > - cmp %edi, %esi > + movl VMX_spec_ctrl(%_ASM_DI), %eax > + cmp PER_CPU_VAR(x86_spec_ctrl_current), %eax Huh. There's a pre-existing bug lurking here, and in the SVM code. SPEC_CTRL is an MSR, i.e. a 64-bit value, but the assembly code assumes bits 63:32 are always zero. So, while this patch looks good, I'm leaning toward skipping it and going straight to a fix. > je .Lspec_ctrl_done > mov $MSR_IA32_SPEC_CTRL, %ecx > xor %edx, %edx > - mov %edi, %eax > wrmsr > > .Lspec_ctrl_done: > -- > 2.50.1 >
