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From:   Paul Durrant <paul@xen.org>
To:     Paolo Bonzini <pbonzini@redhat.com>,
        Jonathan Corbet <corbet@lwn.net>,
        Sean Christopherson <seanjc@google.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Paul Durrant <paul@xen.org>, Shuah Khan <shuah@kernel.org>,
        kvm@vger.kernel.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-kselftest@vger.kernel.org
Subject: [PATCH v10 10/19] KVM: xen: (re-)initialize shared_info if guest (32/64-bit) mode is set
Date:   Mon,  4 Dec 2023 14:43:25 +0000
Message-Id: <20231204144334.910-11-paul@xen.org>
In-Reply-To: <20231204144334.910-1-paul@xen.org>
References: <20231204144334.910-1-paul@xen.org>
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From: Paul Durrant <pdurrant@amazon.com>

If the shared_info PFN cache has already been initialized then the content
of the shared_info page needs to be (re-)initialized whenever the guest
mode is (re)set.
Setting the guest mode is either done explicitly by the VMM via the
KVM_XEN_ATTR_TYPE_LONG_MODE attribute, or implicitly when the guest writes
the MSR to set up the hypercall page.

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
---
Cc: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: x86@kernel.org

v10:
 - New in this version.
---
 arch/x86/kvm/xen.c | 36 +++++++++++++++++++++++++++++-------
 1 file changed, 29 insertions(+), 7 deletions(-)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 9351b32cfcba..3aa452a71890 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -623,10 +623,20 @@ int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
 		if (!IS_ENABLED(CONFIG_64BIT) && data->u.long_mode) {
 			r = -EINVAL;
 		} else {
+			bool lm = !!data->u.long_mode;
+
 			mutex_lock(&kvm->arch.xen.xen_lock);
-			kvm->arch.xen.long_mode = !!data->u.long_mode;
+			if (kvm->arch.xen.long_mode != lm) {
+				kvm->arch.xen.long_mode = lm;
+
+				/*
+				 * Re-initialize shared_info to put the wallclock in the
+				 * correct place.
+				 */
+				r = kvm->arch.xen.shinfo_cache.active ?
+					kvm_xen_shared_info_init(kvm) : 0;
+			}
 			mutex_unlock(&kvm->arch.xen.xen_lock);
-			r = 0;
 		}
 		break;
 
@@ -648,9 +658,6 @@ int kvm_xen_hvm_set_attr(struct kvm *kvm, struct kvm_xen_hvm_attr *data)
 
 		srcu_read_unlock(&kvm->srcu, idx);
 
-		if (!r && kvm->arch.xen.shinfo_cache.active)
-			r = kvm_xen_shared_info_init(kvm);
-
 		mutex_unlock(&kvm->arch.xen.xen_lock);
 		break;
 	}
@@ -1101,9 +1108,24 @@ int kvm_xen_write_hypercall_page(struct kvm_vcpu *vcpu, u64 data)
 	u32 page_num = data & ~PAGE_MASK;
 	u64 page_addr = data & PAGE_MASK;
 	bool lm = is_long_mode(vcpu);
+	int r = 0;
+
+	mutex_lock(&kvm->arch.xen.xen_lock);
+	if (kvm->arch.xen.long_mode != lm) {
+		kvm->arch.xen.long_mode = lm;
+
+		/*
+		 * Re-initialize shared_info to put the wallclock in the
+		 * correct place.
+		 */
+		if (kvm->arch.xen.shinfo_cache.active &&
+		    kvm_xen_shared_info_init(kvm))
+			r = 1;
+	}
+	mutex_unlock(&kvm->arch.xen.xen_lock);
 
-	/* Latch long_mode for shared_info pages etc. */
-	vcpu->kvm->arch.xen.long_mode = lm;
+	if (r)
+		return r;
 
 	/*
 	 * If Xen hypercall intercept is enabled, fill the hypercall
-- 
2.39.2