Re: [RFC PATCH v1 10/15] KVM: VMX: Use WRMSRNS or its immediate form when available
From: Konrad Rzeszutek Wilk <hidden>
Date: 2025-03-31 20:28:25
Also in:
bpf, kvm, linux-edac, linux-ide, linux-perf-users, linux-pm, lkml, llvm, virtualization, xen-devel
On Mon, Mar 31, 2025 at 01:22:46AM -0700, Xin Li (Intel) wrote:
quoted hunk ↗ jump to hunk
Signed-off-by: Xin Li (Intel) <xin@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?
+ WRITE_EAX_TO_MSR_IA32_SPEC_CTRL .Lspec_ctrl_done: -- 2.49.0