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

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Message-Id: <A41676B6-2E9F-4F8E-B91E-8F9A077A2FA8@gmail.com>
Date:   Mon, 26 Jul 2021 17:01:36 -0700
From:   Nadav Amit <nadav.amit@...il.com>
To:     Reiji Watanabe <reijiw@...gle.com>
Cc:     Sean Christopherson <seanjc@...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 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?