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

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Message-ID: <f63d19a8fe6d14186aecc8fcf777284879441ef6.camel@intel.com>
Date: Thu, 21 Mar 2024 01:17:35 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "Yamahata, Isaku" <isaku.yamahata@...el.com>
CC: "Zhang, Tina" <tina.zhang@...el.com>, "isaku.yamahata@...ux.intel.com"
	<isaku.yamahata@...ux.intel.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>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Yuan, Hang" <hang.yuan@...el.com>, "Aktas,
 Erdem" <erdemaktas@...gle.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "isaku.yamahata@...il.com"
	<isaku.yamahata@...il.com>
Subject: Re: [PATCH v19 059/130] KVM: x86/tdp_mmu: Don't zap private pages for
 unsupported cases

On Tue, 2024-03-19 at 17:56 -0700, Rick Edgecombe wrote:
> > Because TDX supports only WB, we
> > ignore the request for MTRR and lapic page change to not zap
> > private
> > pages on unmapping for those two cases
> 
> Hmm. I need to go back and look at this again. It's not clear from
> the
> description why it is safe for the host to not zap pages if requested
> to. I see why the guest wouldn't want them to be zapped.

Ok, I see now how this works. MTRRs and APIC zapping happen to use the
same function: kvm_zap_gfn_range(). So restricting that function from
zapping private pages has the desired affect. I think it's not ideal
that kvm_zap_gfn_range() silently skips zapping some ranges. I wonder
if we could pass something in, so it's more clear to the caller.

But can these code paths even get reaches in TDX? It sounded like MTRRs
basically weren't supported.