linux-kernel - Re: [PATCH] KVM: VMX: Flush shadow VMCS on emergency reboot

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Message-ID: <Z_0XCXwptNhtI_A_@google.com>
Date: Mon, 14 Apr 2025 18:03:44 -0700
From: Sean Christopherson <seanjc@...gle.com>
To: Chao Gao <chao.gao@...el.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Paolo Bonzini <pbonzini@...hat.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, 
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] KVM: VMX: Flush shadow VMCS on emergency reboot

On Mon, Apr 14, 2025, Chao Gao wrote:
> A related topic is why KVM is flushing VMCSs. I haven't found any explicit
> statement in the SDM indicating that the flush is necessary.
> 
> SDM chapter 26.11 mentions:
> 
> If a logical processor leaves VMX operation, any VMCSs active on that logical
> processor may be corrupted (see below). To prevent such corruption of a VMCS
> that may be used either after a return to VMX operation or on another logical
> processor, software should execute VMCLEAR for that VMCS before executing the
> VMXOFF instruction or removing power from the processor (e.g., as part of a
> transition to the S3 and S4 power states).
> 
> To me, the issue appears to be VMCS corruption after leaving VMX operation and
> the flush is necessary only if you intend to use the VMCS after re-entering VMX
> operation.

The problem is that if the CPU flushes a VMCS from the cache at a later time, for
any reason, then the CPU will write back data to main memory.  The issue isn't
reusing the VMCS, it's reusing the underlying memory.