linux-kernel - Re: [PATCH] KVM: x86: dump_vmcs should not assume GUEST_IA32

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Date:   Thu, 18 Feb 2021 08:35:34 -0800
From:   Sean Christopherson <seanjc@...gle.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     David Edmondson <dme@....org>, linux-kernel@...r.kernel.org,
        Borislav Petkov <bp@...en8.de>,
        Wanpeng Li <wanpengli@...cent.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH] KVM: x86: dump_vmcs should not assume GUEST_IA32_EFER is
 valid

On Thu, Feb 18, 2021, Paolo Bonzini wrote:
> On 18/02/21 13:56, David Edmondson wrote:
> > On Thursday, 2021-02-18 at 12:54:52 +01, Paolo Bonzini wrote:
> > 
> > > On 18/02/21 11:04, David Edmondson wrote:
> > > > When dumping the VMCS, retrieve the current guest value of EFER from
> > > > the kvm_vcpu structure if neither VM_EXIT_SAVE_IA32_EFER or
> > > > VM_ENTRY_LOAD_IA32_EFER is set, which can occur if the processor does
> > > > not support the relevant VM-exit/entry controls.
> > > 
> > > Printing vcpu->arch.efer is not the best choice however.  Could we dump
> > > the whole MSR load/store area instead?
> > 
> > I'm happy to do that, and think that it would be useful, but it won't
> > help with the original problem (which I should have explained more).
> > 
> > If the guest has EFER_LMA set but we aren't using the entry/exit
> > controls, vm_read64(GUEST_IA32_EFER) returns 0, causing dump_vmcs() to
> > erroneously dump the PDPTRs.
> 
> Got it now.  It would sort of help, because while dumping the MSR load/store
> area you could get hold of the real EFER, and use it to decide whether to
> dump the PDPTRs.

EFER isn't guaranteed to be in the load list, either, e.g. if guest and host
have the same desired value.

The proper way to retrieve the effective EFER is to reuse the logic in
nested_vmx_calc_efer(), i.e. look at VM_ENTRY_IA32E_MODE if EFER isn't being
loaded via VMCS.