Re: [PATCH v3] KVM: x86: Fix recording of guest steal time / preempted status

From: David Woodhouse <dwmw2@infradead.org>
To: Paolo Bonzini <pbonzini@redhat.com>, kvm <kvm@vger.kernel.org>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Joao Martins <joao.m.martins@oracle.com>,
	"jmattson@google.com" <jmattson@google.com>,
	"wanpengli@tencent.com" <wanpengli@tencent.com>,
	"seanjc@google.com" <seanjc@google.com>,
	"vkuznets@redhat.com" <vkuznets@redhat.com>,
	"mtosatti@redhat.com" <mtosatti@redhat.com>,
	"joro@8bytes.org" <joro@8bytes.org>,
	karahmed@amazon.com
Subject: Re: [PATCH v3] KVM: x86: Fix recording of guest steal time / preempted status
Date: Fri, 12 Nov 2021 19:44:28 +0000	[thread overview]
Message-ID: <fb5efe2aa6424750d0ed2277f72c44101f4c57ba.camel@infradead.org> (raw)
In-Reply-To: <40d7d808-dce6-a541-18dc-b0c7f4d6586c@redhat.com>

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On Fri, 2021-11-12 at 10:31 +0100, Paolo Bonzini wrote:
> Yes, that's also where I got stuck in my first attempt a few months ago. 
>   I agree that it can be changed to use gfn-to-hva caches, except for 
> the vmcs12->posted_intr_desc_addr and vmcs12->virtual_apic_page_addr.

Let's start with the low-hanging fruit... what are the recommended
tests for this kibnd of thing? I don't think we have kernel self-tests
that exercise this, do we?

As before, this is at
https://git.infradead.org/users/dwmw2/linux.git/shortlog/refs/heads/xen-evtchn


From: David Woodhouse <dwmw@amazon.co.uk>
Subject: [PATCH] KVM: nVMX: Use kvm_{read,write}_guest_cached() for shadow_vmcs12

Using kvm_vcpu_map() for reading from the guest is entirely gratuitous,
when all we do is a single memcpy and unmap it again. Fix it up to use
kvm_read_guest()... but in fact I couldn't bring myself to do that
without also making it use a gfn_to_hva_cache for both that *and* the
copy in the other direction.

Signed-off-by: David Woodhouse <dwmw@amazon.co.uk>
---
 arch/x86/kvm/vmx/nested.c | 24 +++++++++++++++---------
 arch/x86/kvm/vmx/vmx.h    |  5 +++++
 2 files changed, 20 insertions(+), 9 deletions(-)

diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
index b213ca966d41..7e2a99f435b6 100644
--- a/arch/x86/kvm/vmx/nested.c
+++ b/arch/x86/kvm/vmx/nested.c
@@ -670,33 +670,39 @@ static inline bool nested_vmx_prepare_msr_bitmap(struct kvm_vcpu *vcpu,
 static void nested_cache_shadow_vmcs12(struct kvm_vcpu *vcpu,
 				       struct vmcs12 *vmcs12)
 {
-	struct kvm_host_map map;
-	struct vmcs12 *shadow;
+	struct vcpu_vmx *vmx = to_vmx(vcpu);
+	struct gfn_to_hva_cache *ghc = &vmx->nested.shadow_vmcs12_cache;
 
 	if (!nested_cpu_has_shadow_vmcs(vmcs12) ||
 	    vmcs12->vmcs_link_pointer == INVALID_GPA)
 		return;
 
-	shadow = get_shadow_vmcs12(vcpu);
-
-	if (kvm_vcpu_map(vcpu, gpa_to_gfn(vmcs12->vmcs_link_pointer), &map))
+	if (ghc->gpa != vmcs12->vmcs_link_pointer &&
+	    kvm_gfn_to_hva_cache_init(vcpu->kvm, ghc,
+				      vmcs12->vmcs_link_pointer, VMCS12_SIZE))
 		return;
 
-	memcpy(shadow, map.hva, VMCS12_SIZE);
-	kvm_vcpu_unmap(vcpu, &map, false);
+	kvm_read_guest_cached(vmx->vcpu.kvm, ghc, get_shadow_vmcs12(vcpu),
+			      VMCS12_SIZE);
 }
 
 static void nested_flush_cached_shadow_vmcs12(struct kvm_vcpu *vcpu,
 					      struct vmcs12 *vmcs12)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
+	struct gfn_to_hva_cache *ghc = &vmx->nested.shadow_vmcs12_cache;
 
 	if (!nested_cpu_has_shadow_vmcs(vmcs12) ||
 	    vmcs12->vmcs_link_pointer == INVALID_GPA)
 		return;
 
-	kvm_write_guest(vmx->vcpu.kvm, vmcs12->vmcs_link_pointer,
-			get_shadow_vmcs12(vcpu), VMCS12_SIZE);
+	if (ghc->gpa != vmcs12->vmcs_link_pointer &&
+	    kvm_gfn_to_hva_cache_init(vcpu->kvm, ghc,
+				      vmcs12->vmcs_link_pointer, VMCS12_SIZE))
+		return;
+
+	kvm_write_guest_cached(vmx->vcpu.kvm, ghc, get_shadow_vmcs12(vcpu),
+			       VMCS12_SIZE);
 }
 
 /*
diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
index a4ead6023133..cdadbd5dc0ca 100644
--- a/arch/x86/kvm/vmx/vmx.h
+++ b/arch/x86/kvm/vmx/vmx.h
@@ -141,6 +141,11 @@ struct nested_vmx {
 	 */
 	struct vmcs12 *cached_shadow_vmcs12;
 
+	/*
+	 * GPA to HVA cache for accessing vmcs12->vmcs_link_pointer
+	 */
+	struct gfn_to_hva_cache shadow_vmcs12_cache;
+
 	/*
 	 * Indicates if the shadow vmcs or enlightened vmcs must be updated
 	 * with the data held by struct vmcs12.
-- 
2.31.1



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