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[92.26.98.202]) by smtp.gmail.com with ESMTPSA id ffacd0b85a97d-39c0b79e0d1sm12187246f8f.70.2025.03.31.13.41.42 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 31 Mar 2025 13:41:43 -0700 (PDT) Message-ID: Date: Mon, 31 Mar 2025 21:41:41 +0100 Precedence: bulk X-Mailing-List: bpf@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH v1 10/15] KVM: VMX: Use WRMSRNS or its immediate form when available To: Konrad Rzeszutek Wilk , "Xin Li (Intel)" Cc: linux-kernel@vger.kernel.org, linux-perf-users@vger.kernel.org, linux-hyperv@vger.kernel.org, virtualization@lists.linux.dev, linux-edac@vger.kernel.org, kvm@vger.kernel.org, xen-devel@lists.xenproject.org, linux-ide@vger.kernel.org, linux-pm@vger.kernel.org, bpf@vger.kernel.org, llvm@lists.linux.dev, tglx@linutronix.de, mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, x86@kernel.org, hpa@zytor.com, jgross@suse.com, peterz@infradead.org, acme@kernel.org, namhyung@kernel.org, mark.rutland@arm.com, alexander.shishkin@linux.intel.com, jolsa@kernel.org, irogers@google.com, adrian.hunter@intel.com, kan.liang@linux.intel.com, wei.liu@kernel.org, ajay.kaher@broadcom.com, alexey.amakhalov@broadcom.com, bcm-kernel-feedback-list@broadcom.com, tony.luck@intel.com, pbonzini@redhat.com, vkuznets@redhat.com, seanjc@google.com, luto@kernel.org, boris.ostrovsky@oracle.com, kys@microsoft.com, haiyangz@microsoft.com, decui@microsoft.com References: <20250331082251.3171276-1-xin@zytor.com> <20250331082251.3171276-11-xin@zytor.com> Content-Language: en-GB From: Andrew Cooper Autocrypt: addr=andrew.cooper3@citrix.com; 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charset=UTF-8 Content-Transfer-Encoding: 7bit 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) >> --- >> 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