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Fri, 14 Nov 2025 07:06:51 -0800 (PST) Received: from [192.168.1.100] ([46.248.82.114]) by smtp.gmail.com with ESMTPSA id a640c23a62f3a-b734fd8099asm408170166b.43.2025.11.14.07.06.50 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 14 Nov 2025 07:06:50 -0800 (PST) Message-ID: <6908a285-b7b7-457a-baaf-fd01c55fe571@gmail.com> Date: Fri, 14 Nov 2025 16:06:49 +0100 Precedence: bulk X-Mailing-List: kvm@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v5 1/9] KVM: VMX: Use on-stack copy of @flags in __vmx_vcpu_run() To: Sean Christopherson , Paolo Bonzini , Thomas Gleixner , Borislav Petkov , Peter Zijlstra , Josh Poimboeuf Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, Pawan Gupta , Brendan Jackman References: <20251113233746.1703361-1-seanjc@google.com> <20251113233746.1703361-2-seanjc@google.com> Content-Language: en-US From: Uros Bizjak In-Reply-To: <20251113233746.1703361-2-seanjc@google.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 11/14/25 00:37, Sean Christopherson wrote: > When testing for VMLAUNCH vs. VMRESUME, use the copy of @flags from the > stack instead of first moving it to EBX, and then propagating > VMX_RUN_VMRESUME to RFLAGS.CF (because RBX is clobbered with the guest > value prior to the conditional branch to VMLAUNCH). Stashing information > in RFLAGS is gross, especially with the writer and reader being bifurcated > by yet more gnarly assembly code. > > Opportunistically drop the SHIFT macros as they existed purely to allow > the VM-Enter flow to use Bit Test. > > Suggested-by: Borislav Petkov > Signed-off-by: Sean Christopherson > --- > arch/x86/kvm/vmx/run_flags.h | 10 +++------- > arch/x86/kvm/vmx/vmenter.S | 13 ++++--------- > 2 files changed, 7 insertions(+), 16 deletions(-) > > diff --git a/arch/x86/kvm/vmx/run_flags.h b/arch/x86/kvm/vmx/run_flags.h > index 2f20fb170def..6a87a12135fb 100644 > --- a/arch/x86/kvm/vmx/run_flags.h > +++ b/arch/x86/kvm/vmx/run_flags.h > @@ -2,12 +2,8 @@ > #ifndef __KVM_X86_VMX_RUN_FLAGS_H > #define __KVM_X86_VMX_RUN_FLAGS_H > > -#define VMX_RUN_VMRESUME_SHIFT 0 > -#define VMX_RUN_SAVE_SPEC_CTRL_SHIFT 1 > -#define VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO_SHIFT 2 > - > -#define VMX_RUN_VMRESUME BIT(VMX_RUN_VMRESUME_SHIFT) > -#define VMX_RUN_SAVE_SPEC_CTRL BIT(VMX_RUN_SAVE_SPEC_CTRL_SHIFT) > -#define VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO BIT(VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO_SHIFT) > +#define VMX_RUN_VMRESUME BIT(0) > +#define VMX_RUN_SAVE_SPEC_CTRL BIT(1) > +#define VMX_RUN_CLEAR_CPU_BUFFERS_FOR_MMIO BIT(2) > > #endif /* __KVM_X86_VMX_RUN_FLAGS_H */ > diff --git a/arch/x86/kvm/vmx/vmenter.S b/arch/x86/kvm/vmx/vmenter.S > index 574159a84ee9..93cf2ca7919a 100644 > --- a/arch/x86/kvm/vmx/vmenter.S > +++ b/arch/x86/kvm/vmx/vmenter.S > @@ -92,7 +92,7 @@ SYM_FUNC_START(__vmx_vcpu_run) > /* Save @vmx for SPEC_CTRL handling */ > push %_ASM_ARG1 > > - /* Save @flags for SPEC_CTRL handling */ > + /* Save @flags (used for VMLAUNCH vs. VMRESUME and mitigations). */ > push %_ASM_ARG3 > > /* > @@ -101,9 +101,6 @@ SYM_FUNC_START(__vmx_vcpu_run) > */ > push %_ASM_ARG2 > > - /* Copy @flags to EBX, _ASM_ARG3 is volatile. */ > - mov %_ASM_ARG3L, %ebx > - > lea (%_ASM_SP), %_ASM_ARG2 > call vmx_update_host_rsp > > @@ -147,9 +144,6 @@ SYM_FUNC_START(__vmx_vcpu_run) > /* Load @regs to RAX. */ > mov (%_ASM_SP), %_ASM_AX > > - /* Check if vmlaunch or vmresume is needed */ > - bt $VMX_RUN_VMRESUME_SHIFT, %ebx > - > /* Load guest registers. Don't clobber flags. */ > mov VCPU_RCX(%_ASM_AX), %_ASM_CX > mov VCPU_RDX(%_ASM_AX), %_ASM_DX > @@ -173,8 +167,9 @@ SYM_FUNC_START(__vmx_vcpu_run) > /* Clobbers EFLAGS.ZF */ > CLEAR_CPU_BUFFERS > > - /* Check EFLAGS.CF from the VMX_RUN_VMRESUME bit test above. */ > - jnc .Lvmlaunch > + /* Check @flags to see if vmlaunch or vmresume is needed. */ > + testl $VMX_RUN_VMRESUME, WORD_SIZE(%_ASM_SP) > + jz .Lvmlaunch You could use TESTB instead of TESTL in the above code to save 3 bytes of code and some memory bandwidth. Assembler will report unwanted truncation if VMX_RUN_VRESUME ever becomes larger than 255. BR, Uros.