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

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Date:   Mon, 19 Jul 2021 21:37:19 -0700
From:   Reiji Watanabe <reijiw@...gle.com>
To:     Sean Christopherson <seanjc@...gle.com>
Cc:     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@...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 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.

Regards,
Reiji