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

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Date: Mon, 25 Mar 2024 23:21:17 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "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>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>, "Chen, Bo2" <chen.bo@...el.com>,
	"sagis@...gle.com" <sagis@...gle.com>, "isaku.yamahata@...il.com"
	<isaku.yamahata@...il.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Aktas, Erdem" <erdemaktas@...gle.com>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>,
	"sean.j.christopherson@...el.com" <sean.j.christopherson@...el.com>, "Yuan,
 Hang" <hang.yuan@...el.com>, "isaku.yamahata@...ux.intel.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 Mon, 2024-03-25 at 16:10 -0700, Isaku Yamahata wrote:
> > > My understanding is that Sean prefers to exit to userspace when KVM can't handle something,
> > > versus
> > > making up behavior that keeps known guests alive. So I would think we should change this patch
> > > to
> > > only be about not using the zapping roots optimization. Then a separate patch should exit to
> > > userspace on attempt to use MTRRs. And we ignore the APIC one.
> > > 
> > > This is trying to guess what maintainers would want here. I'm less sure what Paolo prefers.
> > 
> > When we hit KVM_MSR_FILTER, the current implementation ignores it and makes it
> > error to guest.  Surely we should make it KVM_EXIT_X86_{RDMSR, WRMSR}, instead.
> > It's aligns with the existing implementation(default VM and SW-protected) and
> > more flexible.
> 
> Something like this for "112/130 KVM: TDX: Handle TDX PV rdmsr/wrmsr hypercall"
> Compile only tested at this point.

Seems reasonable to me. Does QEMU configure a special set of MSRs to filter for TDX currently?