Re: [PATCH 2/7] KVM: nVMX: Intercept VMWRITEs to GUEST_{CS,SS}_AR_BYTES

From: Paolo Bonzini <pbonzini@redhat.com>
To: "Sean Christopherson" <sean.j.christopherson@intel.com>,
	"Radim Krčmář" <rkrcmar@redhat.com>
Cc: kvm@vger.kernel.org, Jim Mattson <jmattson@google.com>,
	Liran Alon <liran.alon@oracle.com>
Subject: Re: [PATCH 2/7] KVM: nVMX: Intercept VMWRITEs to GUEST_{CS,SS}_AR_BYTES
Date: Thu, 6 Jun 2019 15:31:55 +0200	[thread overview]
Message-ID: <def2b7f5-6bbf-600a-9edc-53a15f98b478@redhat.com> (raw)
In-Reply-To: <20190507153629.3681-3-sean.j.christopherson@intel.com>

On 07/05/19 17:36, Sean Christopherson wrote:
> Since the AR_BYTES emulation is done only for intercepted VMWRITE, if a
> future patch (re)exposed AR_BYTES for both VMWRITE and VMREAD, then KVM
> would end up with incosistent behavior on pre-Haswell hardware, e.g. KVM
> would drop the reserved bits on intercepted VMWRITE, but direct VMWRITE
> to the shadow VMCS would not drop the bits.

Whoever gets that WARN will have probably a hard time finding again all of this, so:

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index cd51ef68434e..8c5614957e04 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -93,7 +93,7 @@ static void init_vmcs_shadow_fields(void)
 
 		WARN_ONCE(field >= GUEST_ES_AR_BYTES &&
 			  field <= GUEST_TR_AR_BYTES,
-			  "Update vmcs12_write_any() to expose AR_BYTES RW");
+			  "Update vmcs12_write_any() to drop reserved bits from AR_BYTES");
 
 		/*
 		 * PML and the preemption timer can be emulated, but the


Paolo
