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 11:55:45 -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 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.

> Split the CR0 checks for 64-bit vs. 32-bit PAE into separate paths.  The
> 64-bit path is specifically checking state when paging is toggled on,
> i.e. CR0.PG transititions from 0->1.  The PDPTR path now needs to run if
> the new CR0 state has paging enabled, irrespective of whether paging was
> already enabled.  Trying to shave a few cycles to make the PDPTR path an
> "else if" case is a mess.
>
> Fixes: d42e3fae6faed ("kvm: x86: Read PDPTEs on CR0.CD and CR0.NW changes")
> Cc: Jim Mattson <jmattson@...gle.com>
> Cc: Oliver Upton <oupton@...gle.com>
> Cc: Peter Shier <pshier@...gle.com>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
Reviewed-by: Jim Mattson <jmattson@...gle.com>