linux-kernel - Re: [PATCH v2 4/8] KVM: VMX: Optimize vmx_set

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Message-ID: <87muem40wi.fsf@vitty.brq.redhat.com>
Date:   Mon, 30 Sep 2019 10:57:17 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Cc:     Sean Christopherson <sean.j.christopherson@...el.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, Reto Buerki <reet@...elabs.ch>,
        Liran Alon <liran.alon@...cle.com>
Subject: Re: [PATCH v2 4/8] KVM: VMX: Optimize vmx_set_rflags() for unrestricted guest

Sean Christopherson <sean.j.christopherson@...el.com> writes:

> Rework vmx_set_rflags() to avoid the extra code need to handle emulation
> of real mode and invalid state when unrestricted guest is disabled.  The
> primary reason for doing so is to avoid the call to vmx_get_rflags(),
> which will incur a VMREAD when RFLAGS is not already available.  When
> running nested VMs, the majority of calls to vmx_set_rflags() will occur
> without an associated vmx_get_rflags(), i.e. when stuffing GUEST_RFLAGS
> during transitions between vmcs01 and vmcs02.
>
> Note, vmx_get_rflags() guarantees RFLAGS is marked available.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> ---
>  arch/x86/kvm/vmx/vmx.c | 28 ++++++++++++++++++----------
>  1 file changed, 18 insertions(+), 10 deletions(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 83fe8b02b732..814d3e6d0264 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1426,18 +1426,26 @@ unsigned long vmx_get_rflags(struct kvm_vcpu *vcpu)
>  void vmx_set_rflags(struct kvm_vcpu *vcpu, unsigned long rflags)
>  {
>  	struct vcpu_vmx *vmx = to_vmx(vcpu);
> -	unsigned long old_rflags = vmx_get_rflags(vcpu);
> +	unsigned long old_rflags;
>  
> -	__set_bit(VCPU_EXREG_RFLAGS, (ulong *)&vcpu->arch.regs_avail);
> -	vmx->rflags = rflags;
> -	if (vmx->rmode.vm86_active) {
> -		vmx->rmode.save_rflags = rflags;
> -		rflags |= X86_EFLAGS_IOPL | X86_EFLAGS_VM;
> +	if (enable_unrestricted_guest) {
> +		__set_bit(VCPU_EXREG_RFLAGS, (ulong *)&vcpu->arch.regs_avail);
> +
> +		vmx->rflags = rflags;
> +		vmcs_writel(GUEST_RFLAGS, rflags);
> +	} else {
> +		old_rflags = vmx_get_rflags(vcpu);
> +
> +		vmx->rflags = rflags;
> +		if (vmx->rmode.vm86_active) {
> +			vmx->rmode.save_rflags = rflags;
> +			rflags |= X86_EFLAGS_IOPL | X86_EFLAGS_VM;
> +		}
> +		vmcs_writel(GUEST_RFLAGS, rflags);
> +
> +		if ((old_rflags ^ vmx->rflags) & X86_EFLAGS_VM)
> +			vmx->emulation_required = emulation_required(vcpu);
>  	}
> -	vmcs_writel(GUEST_RFLAGS, rflags);

We're doing vmcs_writel() in both branches so it could've stayed here, right?

> -
> -	if ((old_rflags ^ vmx->rflags) & X86_EFLAGS_VM)
> -		vmx->emulation_required = emulation_required(vcpu);
>  }
>  
>  u32 vmx_get_interrupt_shadow(struct kvm_vcpu *vcpu)

Reviewed-by: Vitaly Kuznetsov <vkuznets@...hat.com>

-- 
Vitaly