linux-kernel - Re: [PATCH v2 46/46] KVM: x86: Preserve guest's CR0.CD/NW on INIT

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Date:   Wed, 28 Jul 2021 20:44:33 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Nadav Amit <nadav.amit@...il.com>
Cc:     Reiji Watanabe <reijiw@...gle.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, KVM <kvm@...r.kernel.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 46/46] KVM: x86: Preserve guest's CR0.CD/NW on INIT

On Mon, Jul 26, 2021, Nadav Amit wrote:
> 
> > On Jul 19, 2021, at 9:37 PM, Reiji Watanabe <reijiw@...gle.com> wrote:
> > 
> > On Tue, Jul 13, 2021 at 9:35 AM Sean Christopherson <seanjc@...gle.com> wrote:
> >> 
> >> Preserve CR0.CD and CR0.NW on INIT instead of forcing them to '1', as
> >> defined by both Intel's SDM and AMD's APM.
> >> 
> >> Note, current versions of Intel's SDM are very poorly written with
> >> respect to INIT behavior.  Table 9-1. "IA-32 and Intel 64 Processor
> >> States Following Power-up, Reset, or INIT" quite clearly lists power-up,
> >> RESET, _and_ INIT as setting CR0=60000010H, i.e. CD/NW=1.  But the SDM
> >> then attempts to qualify CD/NW behavior in a footnote:
> >> 
> >>  2. The CD and NW flags are unchanged, bit 4 is set to 1, all other bits
> >>     are cleared.
> >> 
> >> Presumably that footnote is only meant for INIT, as the RESET case and
> >> especially the power-up case are rather non-sensical.  Another footnote
> >> all but confirms that:
> >> 
> >>  6. Internal caches are invalid after power-up and RESET, but left
> >>     unchanged with an INIT.
> >> 
> >> Bare metal testing shows that CD/NW are indeed preserved on INIT (someone
> >> else can hack their BIOS to check RESET and power-up :-D).
> >> 
> >> Reported-by: Reiji Watanabe <reijiw@...gle.com>
> >> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> > 
> > Reviewed-by: Reiji Watanabe <reijiw@...gle.com>
> > 
> > Thank you for the fix and checking the CD/NW with the bare metal testing.
> 
> Interesting.
> 
> Is there a kvm-unit-test to reproduce the issue by any chance?

No :-/