linux-kernel - Re: [PATCH v2] x86/kvm: Force legacy PCI hole to UC when overriding MTRRs for TDX/SNP

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Message-ID: <aL84ElCqFIRF05JM@google.com>
Date: Mon, 8 Sep 2025 13:09:54 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Binbin Wu <binbin.wu@...ux.intel.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Peter Gonda <pgonda@...gle.com>, Vitaly Kuznetsov <vkuznets@...hat.com>, 
	Tom Lendacky <thomas.lendacky@....com>, 
	"Jürgen Groß" <jgross@...e.com>, Korakit Seemakhupt <korakit@...gle.com>, Jianxiong Gao <jxgao@...gle.com>, 
	Nikolay Borisov <nik.borisov@...e.com>
Subject: Re: [PATCH v2] x86/kvm: Force legacy PCI hole to UC when overriding
 MTRRs for TDX/SNP

On Thu, Aug 28, 2025, Binbin Wu wrote:
> > Note #2, relying on guest firmware to handle this scenario, e.g. by setting
> > virtual MTRRs and then consuming them in Linux, is not a viable option, as
> > the virtual MTRR state is managed by the untrusted hypervisor, and because
> > OVMF at least has stopped programming virtual MTRRs when running as a TDX
> > guest.
> 
> Not sure if it needs to mention that with this option, Linux kernel will set
> CR0.CD=1 when programming MTRRs, which will trigger unexpected #VE in TDX guest.

I don't think it's worth bringing up, because that is a very solvable problem,
and orthogonal to the issues with using the untrusted hypervisor to store/track
the virtual MTRR values.