On 31/03/2025 9:27 pm, Konrad Rzeszutek Wilk wrote: > On Mon, Mar 31, 2025 at 01:22:46AM -0700, Xin Li (Intel) wrote: >> Signed-off-by: Xin Li (Intel) <x...@zytor.com> >> --- >> arch/x86/include/asm/msr-index.h | 6 ++++++ >> arch/x86/kvm/vmx/vmenter.S | 28 ++++++++++++++++++++++++---- >> 2 files changed, 30 insertions(+), 4 deletions(-) >> >> diff --git a/arch/x86/include/asm/msr-index.h >> b/arch/x86/include/asm/msr-index.h >> index e6134ef2263d..04244c3ba374 100644 >> --- a/arch/x86/include/asm/msr-index.h >> +++ b/arch/x86/include/asm/msr-index.h >> @@ -1226,4 +1226,10 @@ >> * a #GP >> */ >> >> +/* Instruction opcode for WRMSRNS supported in binutils >= 2.40 */ >> +#define ASM_WRMSRNS _ASM_BYTES(0x0f,0x01,0xc6) >> + >> +/* Instruction opcode for the immediate form RDMSR/WRMSRNS */ >> +#define ASM_WRMSRNS_RAX _ASM_BYTES(0xc4,0xe7,0x7a,0xf6,0xc0) >> + >> #endif /* _ASM_X86_MSR_INDEX_H */ >> diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S >> index f6986dee6f8c..9fae43723c44 100644 >> --- a/arch/x86/kvm/vmx/vmenter.S >> +++ b/arch/x86/kvm/vmx/vmenter.S >> @@ -64,6 +64,29 @@ >> RET >> .endm >> >> +/* >> + * Write EAX to MSR_IA32_SPEC_CTRL. >> + * >> + * Choose the best WRMSR instruction based on availability. >> + * >> + * Replace with 'wrmsrns' and 'wrmsrns %rax, $MSR_IA32_SPEC_CTRL' once >> binutils support them. >> + */ >> +.macro WRITE_EAX_TO_MSR_IA32_SPEC_CTRL >> + ALTERNATIVE_2 __stringify(mov $MSR_IA32_SPEC_CTRL, %ecx; >> \ >> + xor %edx, %edx; >> \ >> + mov %edi, %eax; >> \ >> + ds wrmsr), >> \ >> + __stringify(mov $MSR_IA32_SPEC_CTRL, %ecx; >> \ >> + xor %edx, %edx; >> \ >> + mov %edi, %eax; >> \ >> + ASM_WRMSRNS), >> \ >> + X86_FEATURE_WRMSRNS, >> \ >> + __stringify(xor %_ASM_AX, %_ASM_AX; >> \ >> + mov %edi, %eax; >> \ >> + ASM_WRMSRNS_RAX; .long MSR_IA32_SPEC_CTRL), >> \ >> + X86_FEATURE_MSR_IMM >> +.endm >> + >> .section .noinstr.text, "ax" >> >> /** >> @@ -123,10 +146,7 @@ SYM_FUNC_START(__vmx_vcpu_run) >> movl PER_CPU_VAR(x86_spec_ctrl_current), %esi >> cmp %edi, %esi >> je .Lspec_ctrl_done >> - mov $MSR_IA32_SPEC_CTRL, %ecx >> - xor %edx, %edx >> - mov %edi, %eax >> - wrmsr > Is that the right path forward? > > That is replace the MSR write to disable speculative execution with a > non-serialized WRMSR? Doesn't that mean the WRMSRNS is speculative?
MSR_SPEC_CTRL is explicitly non-serialising, even with a plain WRMSR. non-serialising != non-speculative. Although WRMSRNS's precise statement on the matter of non-speculativeness is woolly, given an intent to optimise it some more in the future. ~Andrew
