linux-kernel - Re: [PATCH v3 19/24] KVM: x86: Introduce per-VM external cache for splitting

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Message-ID: <dfe5cf919eaa922023da8f28bb6718756ad6eaf8.camel@intel.com>
Date: Thu, 22 Jan 2026 10:33:23 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "Zhao, Yan Y" <yan.y.zhao@...el.com>
CC: "Du, Fan" <fan.du@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"Li, Xiaoyao" <xiaoyao.li@...el.com>, "Hansen, Dave" <dave.hansen@...el.com>,
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	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
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	"x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v3 19/24] KVM: x86: Introduce per-VM external cache for
 splitting

On Thu, 2026-01-22 at 15:49 +0800, Zhao, Yan Y wrote:
> On Thu, Jan 22, 2026 at 03:30:16PM +0800, Huang, Kai wrote:
> > > > 
> > > > > Then we can define a structure which contains DPAMT pages for a given 2M
> > > > > range:
> > > > > 
> > > > > 	struct tdx_dmapt_metadata {
> > > > > 		struct page *page1;
> > > > > 		struct page *page2;
> > > > > 	};
> > > 
> > > Note: we need 4 pages to split a 2MB range, 2 for the new S-EPT page, 2 for the
> > > 2MB guest memory range.
> > 
> > In this proposal the pair for S-EPT is already handled by tdx_alloc_page()
> > (or tdx_alloc_control_page()):
> > 
> >   sp->external_spt = tdx_alloc_page();
> Oh, ok.
> 
> So, in the fault path, sp->external_spt and sp->leaf_level_private are from
> fault cache.
> 
> In the non-vCPU split path, sp->external_spt is from tdx_alloc_page(),
> sp->leaf_level_private is from 2 get_zeroed_page() (or the count is from an
> x86_kvm hook ?)

The idea is we can add two new hooks (e.g., obj_alloc()/obj_free()) to
'kvm_mmu_memory_cache' so that we can just hook tdx_alloc_page() for vcpu-
>arch.mmu_external_spte_cache(), in which way KVM will also call
tdx_alloc_page() when topping up memory cache for the S-EPT.

Ditto for DPAMT pair for the actual TDX guest private memory:

We can provide a helper to allocate a structure which contains a pair of
DPAMT pages.  For split path we call that helper directly for sp-
>leaf_level_private; for the fault path we can have another per-vCPU
'kvm_mmu_memory_cache' for DPAMT pair with obj_alloc() being hooked to
that helper so that KVM will also call that helper when topping up the
DPAMT pair cache.

At least this is my idea.