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

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Date: Thu, 21 Mar 2024 15:59:10 -0700
From: Isaku Yamahata <isaku.yamahata@...el.com>
To: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
Cc: "Yamahata, Isaku" <isaku.yamahata@...el.com>,
	"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 Thu, Mar 21, 2024 at 01:17:35AM +0000,
"Edgecombe, Rick P" <rick.p.edgecombe@...el.com> wrote:

> 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.

We can make the code paths so with the (new) assumption that guest MTRR can
be disabled cleanly.
-- 
Isaku Yamahata <isaku.yamahata@...el.com>