linux-kernel - Re: [FYI PATCH] Revert "KVM: x86/mmu: Zap only TDP MMU leafs in kvm_zap_gfn

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Message-ID: <d6367754-7782-7c29-e756-ac02dbd4520b@redhat.com>
Date:   Mon, 21 Mar 2022 10:13:00 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     Vitaly Kuznetsov <vkuznets@...hat.com>,
        Sean Christopherson <sean.j.christopherson@...el.com>
Subject: Re: [FYI PATCH] Revert "KVM: x86/mmu: Zap only TDP MMU leafs in
 kvm_zap_gfn_range()"

On 3/18/22 17:48, Paolo Bonzini wrote:
> This reverts commit cf3e26427c08ad9015956293ab389004ac6a338e.
> 
> Multi-vCPU Hyper-V guests started crashing randomly on boot with the
> latest kvm/queue and the problem can be bisected the problem to this
> particular patch. Basically, I'm not able to boot e.g. 16-vCPU guest
> successfully anymore. Both Intel and AMD seem to be affected. Reverting
> the commit saves the day.
> 
> Reported-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>

This is not enough, the following is also needed to account
for "KVM: x86/mmu: Defer TLB flush to caller when freeing TDP MMU shadow
pages":

------------------- 8< ----------------
From: Paolo Bonzini <pbonzini@...hat.com>
Subject: [PATCH] kvm: x86/mmu: Flush TLB before zap_gfn_range releases RCU

Since "KVM: x86/mmu: Zap only TDP MMU leafs in kvm_zap_gfn_range()"
is going to be reverted, it's not going to be true anymore that
the zap-page flow does not free any 'struct kvm_mmu_page'.  Introduce
an early flush before tdp_mmu_zap_leafs() returns, to preserve
bisectability.

Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index af60922906ef..7f63e1a704e3 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -941,13 +941,17 @@ static bool tdp_mmu_zap_leafs(struct kvm *kvm, struct kvm_mmu_page *root,
  		flush = true;
  	}
  
+	/*
+	 * Need to flush before releasing RCU.  TODO: do it only if intermediate
+	 * page tables were zapped; there is no need to flush under RCU protection
+	 * if no 'struct kvm_mmu_page' is freed.
+	 */
+	if (flush)
+		kvm_flush_remote_tlbs_with_address(kvm, start, end - start);
+
  	rcu_read_unlock();
  
-	/*
-	 * Because this flow zaps _only_ leaf SPTEs, the caller doesn't need
-	 * to provide RCU protection as no 'struct kvm_mmu_page' will be freed.
-	 */
-	return flush;
+	return false;
  }
  
  /*