linux-kernel - Re: [PATCH 0/5] KVM: VMX: Fix MMIO Stale Data Mitigation

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Message-ID: <175088969830.721563.9184862048308832493.b4-ty@google.com>
Date: Wed, 25 Jun 2025 15:25:37 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>, Borislav Petkov <bp@...en8.de>, 
	Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH 0/5] KVM: VMX: Fix MMIO Stale Data Mitigation

On Thu, 22 May 2025 18:17:51 -0700, Sean Christopherson wrote:
> Fix KVM's mitigation of the MMIO Stale Data bug, as the current approach
> doesn't actually detect whether or not a guest has access to MMIO.  E.g.
> KVM_DEV_VFIO_FILE_ADD is entirely optional, and obviously only covers VFIO
> devices, and so is a terrible heuristic for "can this vCPU access MMIO?"
> 
> To fix the flaw (hopefully), track whether or not a vCPU has access to MMIO
> based on the MMU it will run with.  KVM already detects host MMIO when
> installing PTEs in order to force host MMIO to UC (EPT bypasses MTRRs), so
> feeding that information into the MMU is rather straightforward.
> 
> [...]

Applied 1-3 to kvm-x86 mmio, and 4-5 to 'kvm-x86 no_assignment' (which is based
on 'irqs' and includes 'mmio' via a merge, to avoid having the mmio changes
depend on the IRQ overhaul).

[1/5] KVM: x86: Avoid calling kvm_is_mmio_pfn() when kvm_x86_ops.get_mt_mask is NULL
      https://github.com/kvm-x86/linux/commit/c126b46e6fa8
[2/5] KVM: x86/mmu: Locally cache whether a PFN is host MMIO when making a SPTE
      https://github.com/kvm-x86/linux/commit/ffe9d7966d01
[3/5] KVM: VMX: Apply MMIO Stale Data mitigation if KVM maps MMIO into the guest
      https://github.com/kvm-x86/linux/commit/83ebe7157483
[4/5] Revert "kvm: detect assigned device via irqbypass manager"
      https://github.com/kvm-x86/linux/commit/ff845e6a84c8
[5/5] VFIO: KVM: x86: Drop kvm_arch_{start,end}_assignment()
      https://github.com/kvm-x86/linux/commit/bbc13ae593e0

--
https://github.com/kvm-x86/kvm-unit-tests/tree/next