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

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Message-ID: <d85c2e1d-93b3-186d-7df4-80ae6aa03618@redhat.com>
Date:   Wed, 20 May 2020 22:14:47 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <sean.j.christopherson@...el.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 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).

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

Paolo

>>  			guest_cr3 = vcpu->arch.cr3;
> 
> The comment that's just below the context is now stale, e.g. replace
> vmcs01.GUEST_CR3 with vmcs.GUEST_CR3.
> 
>> -- 
>> 2.18.2
>>
>>
>