Message-ID: <23b851db-a2a6-a854-94fc-a747f360d509@intel.com>
Date:   Mon, 13 Feb 2023 07:27:33 -0800
MIME-Version: 1.0
User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101
 Thunderbird/102.6.1
Subject: Re: [PATCH v2 2/8] x86/mtrr: support setting MTRR state for software
 defined MTRRs
Content-Language: en-US
To:     Borislav Petkov <bp@alien8.de>, Juergen Gross <jgross@suse.com>
Cc:     linux-kernel@vger.kernel.org, x86@kernel.org,
        lists@nerdbynature.de, mikelley@microsoft.com,
        torvalds@linux-foundation.org,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        "Shutemov, Kirill" <kirill.shutemov@intel.com>
References: <20230209072220.6836-1-jgross@suse.com>
 <20230209072220.6836-3-jgross@suse.com> <Y+ohfE/wICFKO/93@zn.tnic>
 <6257114d-a957-f586-145c-d2a885417360@suse.com> <Y+pRK6a419jenR9R@zn.tnic>
 <Y+pTDFQlX9qNL35z@zn.tnic>
From:   Dave Hansen <dave.hansen@intel.com>
In-Reply-To: <Y+pTDFQlX9qNL35z@zn.tnic>
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 7bit
Precedence: bulk

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.