linux-kernel - Re: [PATCH 0/2] x86/kvm: Force legacy PCI hole as WB under SNP/TDX

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Message-ID: <bff2170d-680a-489a-8f38-605e37624037@suse.com>
Date: Tue, 8 Jul 2025 17:24:50 +0300
From: Nikolay Borisov <nik.borisov@...e.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, Paolo Bonzini <pbonzini@...hat.com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
 Dionna Glaze <dionnaglaze@...gle.com>, Peter Gonda <pgonda@...gle.com>,
 Jürgen Groß <jgross@...e.com>,
 Kirill Shutemov <kirill.shutemov@...ux.intel.com>,
 Vitaly Kuznetsov <vkuznets@...hat.com>, "H . Peter Anvin" <hpa@...or.com>,
 Binbin Wu <binbin.wu@...el.com>, Tom Lendacky <thomas.lendacky@....com>
Subject: Re: [PATCH 0/2] x86/kvm: Force legacy PCI hole as WB under SNP/TDX

On 2/1/25 02:50, Sean Christopherson wrote:
> Attempt to hack around the SNP/TDX guest MTRR disaster by hijacking
> x86_platform.is_untracked_pat_range() to force the legacy PCI hole, i.e.
> memory from TOLUD => 4GiB, as unconditionally writeback.
> 
> TDX in particular has created an impossible situation with MTRRs.  Because
> TDX disallows toggling CR0.CD, TDX enabling decided the easiest solution
> was to ignore MTRRs entirely (because omitting CR0.CD write is obviously
> too simple).
> 
> Unfortunately, under KVM at least, the kernel subtly relies on MTRRs to
> make ACPI play nice with device drivers.  ACPI tries to map ranges it finds
> as WB, which in turn prevents device drivers from mapping device memory as
> WC/UC-.
> 
> For the record, I hate this hack.  But it's the safest approach I can come
> up with.  E.g. forcing ioremap() to always use WB scares me because it's
> possible, however unlikely, that the kernel could try to map non-emulated
> memory (that is presented as MMIO to the guest) as WC/UC-, and silently
> forcing those mappings to WB could do weird things.
> 
> My initial thought was to effectively revert the offending commit and
> skip the cache disabling/enabling, i.e. the problematic CR0.CD toggling,
> but unfortunately OVMF/EDKII has also added code to skip MTRR setup. :-(
> 
> Sean Christopherson (2):
>    x86/mtrr: Return success vs. "failure" from guest_force_mtrr_state()
>    x86/kvm: Override low memory above TOLUD to WB when MTRRs are forced
>      WB
> 
>   arch/x86/include/asm/mtrr.h        |  5 +++--
>   arch/x86/kernel/cpu/mtrr/generic.c | 11 +++++++----
>   arch/x86/kernel/kvm.c              | 31 ++++++++++++++++++++++++++++--
>   3 files changed, 39 insertions(+), 8 deletions(-)
> 
> 
> base-commit: fd8c09ad0d87783b9b6a27900d66293be45b7bad

This prevents TPM from functioning which in turn breaks attestation on 
TDX enabled guests. So what's the status of it?