linux-kernel - Re: [PATCH v19 059/130] KVM: x86/tdp_mmu: Don't zap private pages for unsupported cases

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Date: Tue, 16 Apr 2024 23:47:06 -0700
From: Isaku Yamahata <isaku.yamahata@...el.com>
To: Chao Gao <chao.gao@...el.com>
Cc: isaku.yamahata@...el.com, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, isaku.yamahata@...il.com,
	Paolo Bonzini <pbonzini@...hat.com>, erdemaktas@...gle.com,
	Sean Christopherson <seanjc@...gle.com>,
	Sagi Shahar <sagis@...gle.com>, Kai Huang <kai.huang@...el.com>,
	chen.bo@...el.com, hang.yuan@...el.com, tina.zhang@...el.com,
	Sean Christopherson <sean.j.christopherson@...el.com>,
	isaku.yamahata@...ux.intel.com
Subject: Re: [PATCH v19 059/130] KVM: x86/tdp_mmu: Don't zap private pages
 for unsupported cases

On Wed, Apr 17, 2024 at 10:21:16AM +0800,
Chao Gao <chao.gao@...el.com> wrote:

> On Mon, Feb 26, 2024 at 12:26:01AM -0800, isaku.yamahata@...el.com wrote:
> >@@ -779,6 +780,10 @@ static bool tdp_mmu_zap_leafs(struct kvm *kvm, struct kvm_mmu_page *root,
> > 
> > 	lockdep_assert_held_write(&kvm->mmu_lock);
> > 
> >+	WARN_ON_ONCE(zap_private && !is_private_sp(root));
> >+	if (!zap_private && is_private_sp(root))
> >+		return false;
> 
> Should be "return flush;".
> 
> Fengwei and I spent one week chasing a bug where virtio-net in the TD guest may
> stop working at some point after bootup if the host enables numad. We finally
> found that the bug was introduced by the 'return false' statement, which left
> some stale EPT entries unflushed.

Thank you for chasing it down.


> I am wondering if we can refactor related functions slightly to make it harder
> to make such mistakes and make it easier to identify them. e.g., we could make
> "@flush" an in/out parameter of tdp_mmu_zap_leafs(), kvm_tdp_mmu_zap_leafs()
> and kvm_tdp_mmu_unmap_gfn_range(). It looks more apparent that "*flush = false"
> below could be problematic if the changes were something like:
> 
> 	if (!zap_private && is_private_sp(root)) {
> 		*flush = false;
> 		return;
> 	}

Yes, let me look into it.
-- 
Isaku Yamahata <isaku.yamahata@...el.com>