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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 v12 19/20] KVM: xen: allow vcpu_info content to be 'safely' copied
Date: Mon, 15 Jan 2024 12:57:06 +0000
Message-Id: <20240115125707.1183-20-paul@xen.org>
In-Reply-To: <20240115125707.1183-1-paul@xen.org>
References: <20240115125707.1183-1-paul@xen.org>
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From: Paul Durrant <pdurrant@amazon.com>

If the guest sets an explicit vcpu_info GPA then, for any of the first 32
vCPUs, the content of the default vcpu_info in the shared_info page must be
copied into the new location. Because this copy may race with event
delivery (which updates the 'evtchn_pending_sel' field in vcpu_info) there
needs to be a way to defer that until the copy is complete.
Happily there is already a shadow of 'evtchn_pending_sel' in kvm_vcpu_xen
that is used in atomic context if the vcpu_info PFN cache has been
invalidated so that the update of vcpu_info can be deferred until the
cache can be refreshed (on vCPU thread's the way back into guest context).

Also use this shadow if the vcpu_info cache has been *deactivated*, so that
the VMM can safely copy the vcpu_info content and then re-activate the
cache with the new GPA. To do this, stop considering an inactive vcpu_info
cache as a hard error in kvm_xen_set_evtchn_fast().

Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
---
Cc: David Woodhouse <dwmw2@infradead.org>
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: x86@kernel.org

v8:
 - Update commit comment.

v6:
 - New in this version.
---
 arch/x86/kvm/xen.c | 3 ---
 1 file changed, 3 deletions(-)

diff --git a/arch/x86/kvm/xen.c b/arch/x86/kvm/xen.c
index 9168a6ec88fd..972098ec4f6a 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -1797,9 +1797,6 @@ int kvm_xen_set_evtchn_fast(struct kvm_xen_evtchn *xe, struct kvm *kvm)
 		WRITE_ONCE(xe->vcpu_idx, vcpu->vcpu_idx);
 	}
 
-	if (!vcpu->arch.xen.vcpu_info_cache.active)
-		return -EINVAL;
-
 	if (xe->port >= max_evtchn_port(kvm))
 		return -EINVAL;
 
-- 
2.39.2