linux-kernel - Re: [PATCH v2 2/8] x86/mtrr: support setting MTRR state for software defined MTRRs

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Date:   Mon, 13 Feb 2023 07:27:33 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Borislav Petkov <bp@...en8.de>, Juergen Gross <jgross@...e.com>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        lists@...dbynature.de, mikelley@...rosoft.com,
        torvalds@...ux-foundation.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "H. Peter Anvin" <hpa@...or.com>,
        "Shutemov, Kirill" <kirill.shutemov@...el.com>
Subject: Re: [PATCH v2 2/8] x86/mtrr: support setting MTRR state for software
 defined MTRRs

On 2/13/23 07:11, Borislav Petkov wrote:
> On Mon, Feb 13, 2023 at 04:03:07PM +0100, Borislav Petkov wrote:
>>> Wouldn't !cpu_feature_enabled(X86_FEATURE_HYPERVISOR) be enough?
>>>
>>> I'm not sure we won't need that for TDX guests, too.
>> See, that's the problem. I wanna have it simple too. Lemme check with
>> dhansen.
> He says MTRRs are enabled in TDX guests: "X86_FEATURE_MTRR is fixed to
> 1 in TDX guests."
> 
> So we will have to do the more finer-grained check I guess.

Yes, TDX guests see MTRRs as being supported.  But, the TDX module also
appears to inject a #VE for all RDMSR or WRMSR's to the MTRRs.  That
makes them effectively useless.

I actually don't know what the heck TDX guests are supposed to do if
they feel like mucking with the MSRs.  The architecture (CPUID) is
essentially telling them: "Sure, go ahead MTRRs are fiiiiiiine".  But
the TDX module is sitting there throwing exceptions (#VE) if the guest
tries to touch MTRRs.

It sounds like there are some guest<->host ABIs on Xen to help the
guests do this.  But I don't see anything in the TDX "GHCI" about it.