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In-Reply-To: <20210527212502.1061857-1-sathyanarayanan.kuppuswamy@linux.intel.com>
From:   Dan Williams <dan.j.williams@intel.com>
Date:   Tue, 8 Jun 2021 16:14:08 -0700
Message-ID: <CAPcyv4gQ6jpphiLn_q3g8c5_bwfywa1P+UfPRibTTzaWAGkBtA@mail.gmail.com>
Subject: Re: [RFC v2-fix-v4 1/1] x86/boot: Avoid #VE during boot for TDX platforms
To:     Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@linux.intel.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        Andy Lutomirski <luto@kernel.org>,
        Dave Hansen <dave.hansen@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Kirill Shutemov <kirill.shutemov@linux.intel.com>,
        Kuppuswamy Sathyanarayanan <knsathya@kernel.org>,
        Raj Ashok <ashok.raj@intel.com>,
        Sean Christopherson <seanjc@google.com>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
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On Thu, May 27, 2021 at 2:25 PM Kuppuswamy Sathyanarayanan
<sathyanarayanan.kuppuswamy@linux.intel.com> wrote:
>
> From: Sean Christopherson <sean.j.christopherson@intel.com>
>
> There are a few MSRs and control register bits which the kernel
> normally needs to modify during boot. But, TDX disallows
> modification of these registers to help provide consistent
> security guarantees. Fortunately, TDX ensures that these are all
> in the correct state before the kernel loads, which means the
> kernel has no need to modify them.
>
> The conditions to avoid are:
>
>   * Any writes to the EFER MSR
>   * Clearing CR0.NE
>   * Clearing CR3.MCE
>
> This theoretically makes guest boot more fragile. If, for
> instance, EFER was set up incorrectly and a WRMSR was performed,
> it will trigger early exception panic or a triple fault, if it's
> before early exceptions are set up. However, this is likely to
> trip up the guest BIOS long before control reaches the kernel. In
> any case, these kinds of problems are unlikely to occur in
> production environments, and developers have good debug
> tools to fix them quickly.
>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
> Reviewed-by: Andi Kleen <ak@linux.intel.com>
> Signed-off-by: Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>

Looks good to me:

Reviewed-by: Dan Williams <dan.j.williams@intel.com>