linux-kernel - Re: [PATCH 21/24] KVM: x86: always update CR3 in VMCB

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Date:   Fri, 22 May 2020 15:47:11 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        vkuznets@...hat.com, Joerg Roedel <jroedel@...e.de>
Subject: Re: [PATCH 21/24] KVM: x86: always update CR3 in VMCB

On Wed, May 20, 2020 at 10:14:47PM +0200, Paolo Bonzini wrote:
> On 20/05/20 20:22, Sean Christopherson wrote:
> > As an alternative fix, what about marking VCPU_EXREG_CR3 dirty in
> > __set_sregs()?  E.g.
> > 
> > 		/*
> > 		 * Loading vmcs02.GUEST_CR3 is handled by nested VM-Enter, but
> > 		 * it can be explicitly dirtied by KVM_SET_SREGS.
> > 		 */
> > 		if (is_guest_mode(vcpu) &&
> > 		    !test_bit(VCPU_EXREG_CR3, (ulong *)&vcpu->arch.regs_dirty))
> > 
> > There's already a dependency on __set_sregs() doing
> > kvm_register_mark_available() before kvm_mmu_reset_context(), i.e. the
> > code is already a bit kludgy.  The dirty check would make the kludge less
> > subtle and provide explicit documentation.
> 
> A comment in __set_sregs is certainly a good idea.  But checking for
> dirty seems worse since the caching of CR3 is a bit special in this
> respect (it's never marked dirty).

That's why I thought it was so clever :-)

> This patch should probably be split too, so that the Fixes tags are
> separate for Intel and AMD.

That would be nice.