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

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Date:   Mon, 30 Sep 2019 17:55:41 +0200
From:   Vitaly Kuznetsov <vkuznets@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...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, 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:

> On Mon, Sep 30, 2019 at 10:57:17AM +0200, Vitaly Kuznetsov wrote:
>> 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?
>
> Yes, but the resulting code is a bit ugly.  emulation_required() consumes
> vmcs.GUEST_RFLAGS, i.e. the if statement that reads old_rflags would also
> need to be outside of the else{} case.  
>
> This isn't too bad:
>
> 	if (!enable_unrestricted_guest && 
> 	    ((old_rflags ^ vmx->rflags) & X86_EFLAGS_VM))
> 		vmx->emulation_required = emulation_required(vcpu);
>
> but gcc isn't smart enough to understand old_rflags won't be used if
> enable_unrestricted_guest, so old_rflags either needs to be tagged with
> uninitialized_var() or explicitly initialized in the if(){} case.
>
> Duplicating a small amount of code felt like the lesser of two evils.
>

I see, thanks for these additional details!

-- 
Vitaly