linux-kernel - Re: [PATCH 16/21] KVM: TDX: Premap initial guest memory

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Message-ID: <2f311f763092f6e462061f6cd55b8633379486bc.camel@intel.com>
Date: Wed, 11 Sep 2024 00:30:40 +0000
From: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>
To: "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "pbonzini@...hat.com"
	<pbonzini@...hat.com>, "seanjc@...gle.com" <seanjc@...gle.com>
CC: "Zhao, Yan Y" <yan.y.zhao@...el.com>, "nik.borisov@...e.com"
	<nik.borisov@...e.com>, "dmatlack@...gle.com" <dmatlack@...gle.com>, "Huang,
 Kai" <kai.huang@...el.com>, "isaku.yamahata@...il.com"
	<isaku.yamahata@...il.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 16/21] KVM: TDX: Premap initial guest memory

On Tue, 2024-09-10 at 12:49 +0200, Paolo Bonzini wrote:
> On 9/4/24 05:07, Rick Edgecombe wrote:
> > From: Isaku Yamahata <isaku.yamahata@...el.com>
> > 
> > Update TDX's hook of set_external_spte() to record pre-mapping cnt instead
> > of doing nothing and returning when TD is not finalized.
> > 
> > TDX uses ioctl KVM_TDX_INIT_MEM_REGION to initialize its initial guest
> > memory. This ioctl calls kvm_gmem_populate() to get guest pages and in
> > tdx_gmem_post_populate(), it will
> > (1) Map page table pages into KVM mirror page table and private EPT.
> > (2) Map guest pages into KVM mirror page table. In the propagation hook,
> >       just record pre-mapping cnt without mapping the guest page into
> > private
> >       EPT.
> > (3) Map guest pages into private EPT and decrease pre-mapping cnt.
> > 
> > Do not map guest pages into private EPT directly in step (2), because TDX
> > requires TDH.MEM.PAGE.ADD() to add a guest page before TD is finalized,
> > which copies page content from a source page from user to target guest page
> > to be added. However, source page is not available via common interface
> > kvm_tdp_map_page() in step (2).
> > 
> > Therefore, just pre-map the guest page into KVM mirror page table and
> > record the pre-mapping cnt in TDX's propagation hook. The pre-mapping cnt
> > would be decreased in ioctl KVM_TDX_INIT_MEM_REGION when the guest page is
> > mapped into private EPT.
> 
> Stale commit message; squashing all of it into patch 20 is an easy cop 
> out...

Arh, yes this has details that are not relevant to the patch.

Squashing it seems fine, but I wasn't sure about whether we actually needed this
nr_premapped. It was one of the things we decided to punt a decision on in order
to continue our debates on the list. So we need to pick up the debate again.