linux-kernel - Re: [RFC PATCH v5 19/45] KVM: Allow owner of kvm_mmu_memory

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Message-ID: <aYJWvKagesT3FPfI@google.com>
Date: Tue, 3 Feb 2026 12:12:44 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Kai Huang <kai.huang@...el.com>
Cc: "x86@...nel.org" <x86@...nel.org>, 
	"dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>, "kas@...nel.org" <kas@...nel.org>, 
	"bp@...en8.de" <bp@...en8.de>, "mingo@...hat.com" <mingo@...hat.com>, 
	"pbonzini@...hat.com" <pbonzini@...hat.com>, "tglx@...nel.org" <tglx@...nel.org>, 
	Rick P Edgecombe <rick.p.edgecombe@...el.com>, 
	"ackerleytng@...gle.com" <ackerleytng@...gle.com>, "sagis@...gle.com" <sagis@...gle.com>, 
	Vishal Annapurve <vannapurve@...gle.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, Yan Y Zhao <yan.y.zhao@...el.com>, 
	Xiaoyao Li <xiaoyao.li@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>, 
	"linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>, Isaku Yamahata <isaku.yamahata@...el.com>, 
	"binbin.wu@...ux.intel.com" <binbin.wu@...ux.intel.com>
Subject: Re: [RFC PATCH v5 19/45] KVM: Allow owner of kvm_mmu_memory_cache to
 provide a custom page allocator

On Tue, Feb 03, 2026, Kai Huang wrote:
> On Wed, 2026-01-28 at 17:14 -0800, Sean Christopherson wrote:
> > Extend "struct kvm_mmu_memory_cache" to support a custom page allocator
> > so that x86's TDX can update per-page metadata on allocation and free().
> > 
> > Name the allocator page_get() to align with __get_free_page(), e.g. to
> > communicate that it returns an "unsigned long", not a "struct page", and
> > to avoid collisions with macros, e.g. with alloc_page.
> > 
> > Suggested-by: Kai Huang <kai.huang@...el.com>
> > Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> 
> I thought it could be more generic for allocating an object, but not just a
> page.
> 
> E.g., I thought we might be able to use it to allocate a structure which has
> "pair of DPAMT pages" so it could be assigned to 'struct kvm_mmu_page'.  But
> it seems you abandoned this idea.  May I ask why?  Just want to understand
> the reasoning here.

Because that requires more complexity and there's no known use case, and I don't
see an obvious way for a use case to come along.  All of the motiviations for a
custom allocation scheme that I can think of apply only to full pages, or fit
nicely in a kmem_cache.

Specifically, the "cache" logic is already bifurcated between "kmem_cache' and
"page" usage.  Further splitting the "page" case doesn't require modifications to
the "kmem_cache" case, whereas providing a fully generic solution would require
additional changes, e.g. to handle this code:

	page = (void *)__get_free_page(gfp_flags);
	if (page && mc->init_value)
		memset64(page, mc->init_value, PAGE_SIZE / sizeof(u64));

It certainly wouldn't be much complexity, but this code is already a bit awkward,
so I don't think it makes sense to add support for something that will probably
never be used.