linux-kernel - Re: [PATCH v3 7/8] KVM: x86: nSVM: Handle restore of legacy nested state

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Message-ID: <aYY-ZwjEnHbY5J-T@google.com>
Date: Fri, 6 Feb 2026 11:17:59 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Jim Mattson <jmattson@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Thomas Gleixner <tglx@...nel.org>, Ingo Molnar <mingo@...hat.com>, 
	Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org, 
	"H. Peter Anvin" <hpa@...or.com>, Shuah Khan <shuah@...nel.org>, kvm@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	Yosry Ahmed <yosry.ahmed@...ux.dev>
Subject: Re: [PATCH v3 7/8] KVM: x86: nSVM: Handle restore of legacy nested state

On Thu, Feb 05, 2026, Jim Mattson wrote:
> When nested NPT is enabled and KVM_SET_NESTED_STATE is used to restore an
> old checkpoint (without a valid gPAT), the current IA32_PAT value must be
> used as L2's gPAT.
> 
> The current IA32_PAT value may be restored by KVM_SET_MSRS after
> KVM_SET_NESTED_STATE. Furthermore, there may be a KVM_GET_NESTED_STATE
> before the first KVM_RUN.
> 
> Introduce a new boolean, svm->nested.legacy_gpat_semantics. When set, hPAT
> updates are also applied to gPAT, preserving the old behavior where L2
> shared L1's PAT. svm_vcpu_pre_run() clears this boolean at the first
> KVM_RUN.

State this last point as a command and explain why.  E.g. I think this is why?

  Clear legacy_gpat_semantics on KVM_RUN so that the legacy semantics are
  scoped to a single restore (inasmuch as possible).  E.g. to support
  restoring a snapshot from an old KVM, and then later restoring a snapshot
  from a new KVM.