linux-kernel - Re: [PATCH 42/43] KVM: VMX: Drop VMWRITEs to zero fields at vCPU RESET

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Date:   Mon, 24 May 2021 23:15:07 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 42/43] KVM: VMX: Drop VMWRITEs to zero fields at vCPU
 RESET

On 24/04/21 02:46, Sean Christopherson wrote:
> Don't waste time writing zeros via VMWRITE during vCPU RESET, the VMCS
> is zero allocated.

Is this guaranteed to be valid, or could the VMCS in principle use some 
weird encoding? (Like it does for the access rights, even though this 
does not matter for this patch).

Paolo

> No functional change intended.
> 
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>   arch/x86/kvm/vmx/vmx.c | 29 -----------------------------
>   1 file changed, 29 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 78d17adce7e6..74258ba4832a 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -4427,13 +4427,6 @@ static void init_vmcs(struct vcpu_vmx *vmx)
>   	}
>   
>   	if (kvm_vcpu_apicv_active(&vmx->vcpu)) {
> -		vmcs_write64(EOI_EXIT_BITMAP0, 0);
> -		vmcs_write64(EOI_EXIT_BITMAP1, 0);
> -		vmcs_write64(EOI_EXIT_BITMAP2, 0);
> -		vmcs_write64(EOI_EXIT_BITMAP3, 0);
> -
> -		vmcs_write16(GUEST_INTR_STATUS, 0);
> -
>   		vmcs_write16(POSTED_INTR_NV, POSTED_INTR_VECTOR);
>   		vmcs_write64(POSTED_INTR_DESC_ADDR, __pa((&vmx->pi_desc)));
>   	}
> @@ -4444,23 +4437,9 @@ static void init_vmcs(struct vcpu_vmx *vmx)
>   		vmx->ple_window_dirty = true;
>   	}
>   
> -	vmcs_write32(PAGE_FAULT_ERROR_CODE_MASK, 0);
> -	vmcs_write32(PAGE_FAULT_ERROR_CODE_MATCH, 0);
> -	vmcs_write32(CR3_TARGET_COUNT, 0);           /* 22.2.1 */
> -
> -	vmcs_write16(HOST_FS_SELECTOR, 0);            /* 22.2.4 */
> -	vmcs_write16(HOST_GS_SELECTOR, 0);            /* 22.2.4 */
>   	vmx_set_constant_host_state(vmx);
> -	vmcs_writel(HOST_FS_BASE, 0); /* 22.2.4 */
> -	vmcs_writel(HOST_GS_BASE, 0); /* 22.2.4 */
>   
> -	if (cpu_has_vmx_vmfunc())
> -		vmcs_write64(VM_FUNCTION_CONTROL, 0);
> -
> -	vmcs_write32(VM_EXIT_MSR_STORE_COUNT, 0);
> -	vmcs_write32(VM_EXIT_MSR_LOAD_COUNT, 0);
>   	vmcs_write64(VM_EXIT_MSR_LOAD_ADDR, __pa(vmx->msr_autoload.host.val));
> -	vmcs_write32(VM_ENTRY_MSR_LOAD_COUNT, 0);
>   	vmcs_write64(VM_ENTRY_MSR_LOAD_ADDR, __pa(vmx->msr_autoload.guest.val));
>   
>   	if (vmcs_config.vmentry_ctrl & VM_ENTRY_LOAD_IA32_PAT)
> @@ -4493,7 +4472,6 @@ static void init_vmcs(struct vcpu_vmx *vmx)
>   		memset(&vmx->pt_desc, 0, sizeof(vmx->pt_desc));
>   		/* Bit[6~0] are forced to 1, writes are ignored. */
>   		vmx->pt_desc.guest.output_mask = 0x7F;
> -		vmcs_write64(GUEST_IA32_RTIT_CTL, 0);
>   	}
>   
>   	vmx_setup_uret_msrs(vmx);
> @@ -4536,13 +4514,6 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu, bool init_event)
>   	vmcs_write32(GUEST_LDTR_LIMIT, 0xffff);
>   	vmcs_write32(GUEST_LDTR_AR_BYTES, 0x00082);
>   
> -	if (!init_event) {
> -		vmcs_write32(GUEST_SYSENTER_CS, 0);
> -		vmcs_writel(GUEST_SYSENTER_ESP, 0);
> -		vmcs_writel(GUEST_SYSENTER_EIP, 0);
> -		vmcs_write64(GUEST_IA32_DEBUGCTL, 0);
> -	}
> -
>   	vmcs_writel(GUEST_GDTR_BASE, 0);
>   	vmcs_write32(GUEST_GDTR_LIMIT, 0xffff);
>   
>