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From:   Paul Durrant <paul@xen.org>
To:     kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Cc:     Paul Durrant <pdurrant@amazon.com>,
        David Woodhouse <dwmw@amazon.co.uk>,
        David Woodhouse <dwmw2@infradead.org>,
        Sean Christopherson <seanjc@google.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org
Subject: [PATCH v7 11/11] KVM: xen: allow vcpu_info content to be 'safely' copied
Date:   Mon,  2 Oct 2023 09:57:40 +0000
Message-Id: <20231002095740.1472907-12-paul@xen.org>
In-Reply-To: <20231002095740.1472907-1-paul@xen.org>
References: <20231002095740.1472907-1-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) we
need 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).
So let's 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, all we need to do
is 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

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 aafc794940e4..e645066217bb 100644
--- a/arch/x86/kvm/xen.c
+++ b/arch/x86/kvm/xen.c
@@ -1606,9 +1606,6 @@ 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