linux-kernel - Re: [PATCH 3/4] KVM: x86/mmu: Make sure pfn is not changed for spurious fault

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Message-ID: <Z6bDZWzePT6CAreU@yzhao56-desk.sh.intel.com>
Date: Sat, 8 Feb 2025 10:37:25 +0800
From: Yan Zhao <yan.y.zhao@...el.com>
To: Sean Christopherson <seanjc@...gle.com>
CC: <pbonzini@...hat.com>, <rick.p.edgecombe@...el.com>,
	<linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>
Subject: Re: [PATCH 3/4] KVM: x86/mmu: Make sure pfn is not changed for
 spurious fault

On Fri, Feb 07, 2025 at 07:07:06AM -0800, Sean Christopherson wrote:
> On Fri, Feb 07, 2025, Yan Zhao wrote:
> > Make sure pfn is not changed for a spurious fault by warning in the TDP
> > MMU. For shadow path, only treat a prefetch fault as spurious when pfn is
> > not changed, since the rmap removal and add are required when pfn is
> > changed.
> 
> I like sanity checks, but I don't like special casing "prefetch" faults like this.
> KVM should _never_ change the PFN of a shadow-present SPTE.  The TDP MMU already
> BUG()s on this, and mmu_spte_update() WARNs on the transition.
However, both TDP MMU and mmu_set_spte() return RET_PF_SPURIOUS directly before
the BUG() in TDP MMU or mmu_spte_update() could be hit.