From mboxrd@z Thu Jan 1 00:00:00 1970 From: Lan Tianyu Subject: Re: [PATCH] KVM/VMX: Avoid CR3 VMEXIT during guest real mode when "unrestricted guest" is supported. Date: Thu, 17 Aug 2017 13:00:11 +0800 Message-ID: References: <1502848704-7474-1-git-send-email-tianyu.lan@intel.com> <20170816132625.GD5975@flask> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit Cc: pbonzini@redhat.com, tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com, x86@kernel.org, kvm@vger.kernel.org, linux-kernel@vger.kernel.org To: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= Return-path: In-Reply-To: <20170816132625.GD5975@flask> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 2017年08月16日 21:26, Radim Krčmář wrote: > 2017-08-15 21:58-0400, Lan Tianyu: >> These CR3 VMEXITs was introduced for platform without "unrestricted guest" >> support. This is to set ept identity table to guest CR3 in guest real >> mode because these platforms don't support ept real mode(CR0.PE and CR0.PG >> must be set to 1). But these VMEXITs is redundant for platforms with >> "unrestricted guest" support. >> >> Signed-off-by: Lan Tianyu >> --- >> arch/x86/kvm/vmx.c | 22 +++++++++++++--------- >> 1 file changed, 13 insertions(+), 9 deletions(-) >> >> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c >> @@ -4311,7 +4313,9 @@ static void vmx_set_cr3(struct kvm_vcpu *vcpu, unsigned long cr3) >> } >> >> vmx_flush_tlb(vcpu); >> - vmcs_writel(GUEST_CR3, guest_cr3); >> + >> + if (!enable_unrestricted_guest || !enable_ept) >> + vmcs_writel(GUEST_CR3, guest_cr3); > > This looks wrong -- it would prevent update GUEST_CR3 outside of > non-root mode with enable_unrestricted_guest. > OK. Do you mean nest mode? I didn't consider that case. I thought there were three cases here. 1) Shadow page mode(enable_ept=0) 2) ept mode without unrestricted guest mode (ept=1, enable_unrestricted_guest = 0) 3) ept mode with unrestricted guest mode (ept=1, enable_unrestricted_guest = 1) >>From my understanding, only (1) and (2) need to update guest cr3. If nest mode is still needed to update guest CR3, we can add is_guest_mode() in the if condition. Other choice is to just ignore setting guest cr3 for case3. The condition maybe changed to if (!(enable_unrestricted_guest && enable_ept)) vmcs_writel(GUEST_CR3, guest_cr3); -- Best regards Tianyu Lan