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

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Date: Wed, 27 Mar 2024 17:36:07 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Li, Xiaoyao" <xiaoyao.li@...el.com>, "Gao, Chao" <chao.gao@...el.com>,
	"Yamahata, Isaku" <isaku.yamahata@...el.com>
CC: "Zhang, Tina" <tina.zhang@...el.com>, "seanjc@...gle.com"
	<seanjc@...gle.com>, "Huang, Kai" <kai.huang@...el.com>,
	"sean.j.christopherson@...el.com" <sean.j.christopherson@...el.com>, "Chen,
 Bo2" <chen.bo@...el.com>, "sagis@...gle.com" <sagis@...gle.com>,
	"isaku.yamahata@...ux.intel.com" <isaku.yamahata@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "Aktas, Erdem"
	<erdemaktas@...gle.com>, "isaku.yamahata@...il.com"
	<isaku.yamahata@...il.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"Yuan, Hang" <hang.yuan@...el.com>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>
Subject: Re: [PATCH v19 059/130] KVM: x86/tdp_mmu: Don't zap private pages for
 unsupported cases

On Wed, 2024-03-27 at 10:54 +0800, Xiaoyao Li wrote:
> > > If QEMU doesn't configure the msr filter list correctly, KVM has to handle
> > > guest's MTRR MSR accesses. In my understanding, the suggestion is KVM zap
> > > private memory mappings. But guests won't accept memory again because no one
> > > currently requests guests to do this after writes to MTRR MSRs. In this case,
> > > guests may access unaccepted memory, causing infinite EPT violation loop
> > > (assume SEPT_VE_DISABLE is set). This won't impact other guests/workloads on
> > > the host. But I think it would be better if we can avoid wasting CPU resource
> > > on the useless EPT violation loop.
> > 
> > Qemu is expected to do it correctly.  There are manyways for userspace to go
> > wrong.  This isn't specific to MTRR MSR.
> 
> This seems incorrect. KVM shouldn't force userspace to filter some 
> specific MSRs. The semantic of MSR filter is userspace configures it on 
> its own will, not KVM requires to do so.

I'm ok just always doing the exit to userspace on attempt to use MTRRs in a TD, and not rely on the
MSR list. At least I don't see the problem.