linux-kernel - [PATCH] KVM: x86/MMU: Zap all when removing memslot if VM has assigned device

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Message-Id: <20190815151228.32242-1-sean.j.christopherson@intel.com>
Date:   Thu, 15 Aug 2019 08:12:28 -0700
From:   Sean Christopherson <sean.j.christopherson@...el.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        Alex Willamson <alex.williamson@...hat.com>
Subject: [PATCH] KVM: x86/MMU: Zap all when removing memslot if VM has assigned device

Alex Williamson reported regressions with device assignment when KVM
changed its memslot removal logic to zap only the SPTEs for the memslot
being removed.  The source of the bug is unknown at this time, and root
causing the issue will likely be a slow process.  In the short term, fix
the regression by zapping all SPTEs when removing a memslot from a VM
with assigned device(s).

Fixes: 4e103134b862 ("KVM: x86/mmu: Zap only the relevant pages when removing a memslot", 2019-02-05)
Reported-by: Alex Willamson <alex.williamson@...hat.com>
Cc: stable@...r.kernel.org
Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
---

An alternative idea to a full revert.  I assume this would be easy to
backport, and also easy to revert or quirk depending on where the bug
is hiding.

 arch/x86/kvm/mmu.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
index 8f72526e2f68..358b93882ac6 100644
--- a/arch/x86/kvm/mmu.c
+++ b/arch/x86/kvm/mmu.c
@@ -5659,6 +5659,17 @@ static void kvm_mmu_invalidate_zap_pages_in_memslot(struct kvm *kvm,
 	bool flush;
 	gfn_t gfn;
 
+	/*
+	 * Zapping only the removed memslot introduced regressions for VMs with
+	 * assigned devices.  It is unknown what piece of code is buggy.  Until
+	 * the source of the bug is identified, zap everything if the VM has an
+	 * assigned device.
+	 */
+	if (kvm_arch_has_assigned_device(kvm)) {
+		kvm_mmu_zap_all(kvm);
+		return;
+	}
+
 	spin_lock(&kvm->mmu_lock);
 
 	if (list_empty(&kvm->arch.active_mmu_pages))
-- 
2.22.0