linux-kernel - Re: [PATCH 1/2] KVM: nVMX: Always write vmcs02.GUEST

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Message-ID: <20190927142258.GA24889@linux.intel.com>
Date:   Fri, 27 Sep 2019 07:22:58 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Jim Mattson <jmattson@...gle.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Reto Buerki <reet@...elabs.ch>
Subject: Re: [PATCH 1/2] KVM: nVMX: Always write vmcs02.GUEST_CR3 during
 nested VM-Enter

On Thu, Sep 26, 2019 at 04:39:28PM -0700, Jim Mattson wrote:
> On Thu, Sep 26, 2019 at 2:43 PM Sean Christopherson
> <sean.j.christopherson@...el.com> wrote:
> > -       vmcs_writel(GUEST_CR3, guest_cr3);
> > +       if (!skip_cr3)
> > +               vmcs_writel(GUEST_CR3, guest_cr3);
> 
> Is this part of the change necessary, or is it just an optimization to
> save a redundant VMWRITE?

Save the redundant VMWRITE.  I also wanted to convey the idea that the
nested code is responsible for setting GUEST_CR3 to the correct value.
I'd be happy to add a comment to make the latter point explicit.

> >  }
> >
> >  int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
> > --
> > 2.22.0
> >