Re: [PATCH 02/13] KVM: nSVM: don't call nested_sync_control_from_vmcb02 on each VM exit

From: Sean Christopherson <seanjc@google.com>
To: Maxim Levitsky <mlevitsk@redhat.com>
Cc: kvm@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	linux-kernel@vger.kernel.org,
	Peter Zijlstra <peterz@infradead.org>,
	Thomas Gleixner <tglx@linutronix.de>,
	Sandipan Das <sandipan.das@amd.com>,
	Daniel Sneddon <daniel.sneddon@linux.intel.com>,
	Jing Liu <jing2.liu@intel.com>,
	Josh Poimboeuf <jpoimboe@kernel.org>,
	Wyes Karny <wyes.karny@amd.com>, Borislav Petkov <bp@alien8.de>,
	Babu Moger <babu.moger@amd.com>,
	Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
	Jim Mattson <jmattson@google.com>,
	x86@kernel.org
Subject: Re: [PATCH 02/13] KVM: nSVM: don't call nested_sync_control_from_vmcb02 on each VM exit
Date: Thu, 17 Nov 2022 20:04:54 +0000	[thread overview]
Message-ID: <Y3aT5qBgOuwsOeS/@google.com> (raw)
In-Reply-To: <20221117143242.102721-3-mlevitsk@redhat.com>

On Thu, Nov 17, 2022, Maxim Levitsky wrote:
> Calling nested_sync_control_from_vmcb02 on each VM exit (nested or not),
> was an attempt to keep the int_ctl field in the vmcb12 cache
> up to date on each VM exit.

This doesn't mesh with the reasoning in commit 2d8a42be0e2b ("KVM: nSVM: synchronize
VMCB controls updated by the processor on every vmexit"), which states that the
goal is to keep svm->nested.ctl.* synchronized, not vmcb12.  Or is nested.ctl the
cache you are referring to?

> However all other fields in the vmcb12 cache are not kept up to	 date,

IIUC, this isn't technically true.  They are up-to-date because they're never
modified by hardware.

> therefore for consistency it is better to do this on a nested VM exit only.

Again, IIUC, this actually introduces an inconsistency because it leaves stale
state in svm->nested.ctl, whereas the existing code ensures all state in
svm->nested.ctl is fresh immediately after non-nested VM-Exit.