Re: [RFC PATCH v5 19/45] KVM: Allow owner of kvm_mmu_memory_cache to provide a custom page allocator

[Huang, Kai]

On Tue, 2026-02-03 at 12:12 -0800, Sean Christopherson wrote:
> 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@xxxxxxxxx>
> > > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > 
> > 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.

For this particular piece of code, we can add a helper for allocating normal
page table pages, get rid of mc->init_value completely and hook mc-
>page_get() to that helper.

A bonus is we can then call that helper in all places when KVM needs to
allocate a page for normal page table instead of just calling
get_zerod_pages() directly, e.g., like the one in
tdp_mmu_alloc_sp_for_split(), so that we can have a consistent way for
allocating normal page table pages.