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

[Sean Christopherson <seanjc@google.com>]

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@intel.com>
> > Signed-off-by: Sean Christopherson <seanjc@google.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.