linux-kernel - Re: [PATCH] KVM: x86: Don't attempt to load PDPTRs when 64-bit mode is enabled

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Date:   Tue, 14 Jul 2020 12:02:14 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...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>,
        Oliver Upton <oupton@...gle.com>,
        Peter Shier <pshier@...gle.com>
Subject: Re: [PATCH] KVM: x86: Don't attempt to load PDPTRs when 64-bit mode
 is enabled

On Tue, Jul 14, 2020 at 11:59 AM Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> On Tue, Jul 14, 2020 at 11:55:45AM -0700, Jim Mattson wrote:
> > On Mon, Jul 13, 2020 at 6:57 PM Sean Christopherson
> > <sean.j.christopherson@...el.com> wrote:
> > >
> > > Don't attempt to load PDPTRs if EFER.LME=1, i.e. if 64-bit mode is
> > > enabled.  A recent change to reload the PDTPRs when CR0.CD or CR0.NW is
> > > toggled botched the EFER.LME handling and sends KVM down the PDTPR path
> > > when is_paging() is true, i.e. when the guest toggles CD/NW in 64-bit
> > > mode.
> >
> > Oops!
> >
> > I don't think "is_paging()" is relevant here, so much as "EFER.LME=1."
> > As you note below, KVM *should* go down the PDPTR path when
> > is_paging() is true and EFER.LME=0.
>
> It's relevant for the EFER.LME=1 case as it's used to detect CR0.PG 0->1.
>
> Though maybe we're in violent agreement?

We're in agreement conceptually, but I find your original text lacking
in clarity. :-)