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

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Message-ID: <de05853257e9cc66998101943f78a4b7e6e3d741.camel@intel.com>
Date: Tue, 3 Feb 2026 10:56:14 +0000
From: "Huang, Kai" <kai.huang@...el.com>
To: "seanjc@...gle.com" <seanjc@...gle.com>, "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>
CC: "Edgecombe, Rick P" <rick.p.edgecombe@...el.com>, "ackerleytng@...gle.com"
	<ackerleytng@...gle.com>, "sagis@...gle.com" <sagis@...gle.com>, "Annapurve,
 Vishal" <vannapurve@...gle.com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Zhao, Yan Y" <yan.y.zhao@...el.com>, "Li,
 Xiaoyao" <xiaoyao.li@...el.com>, "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>, "Yamahata, Isaku"
	<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 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.

Anyway:

Reviewed-by: Kai Huang <kai.huang@...el.com>

> ---
>  include/linux/kvm_types.h | 2 ++
>  virt/kvm/kvm_main.c       | 7 ++++++-
>  2 files changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/kvm_types.h b/include/linux/kvm_types.h
> index a568d8e6f4e8..87fa9deffdb7 100644
> --- a/include/linux/kvm_types.h
> +++ b/include/linux/kvm_types.h
> @@ -112,6 +112,8 @@ struct kvm_mmu_memory_cache {
>  	gfp_t gfp_custom;
>  	u64 init_value;
>  	struct kmem_cache *kmem_cache;
> +	unsigned long (*page_get)(gfp_t gfp);
> +	void (*page_free)(unsigned long addr);
>  	int capacity;
>  	int nobjs;
>  	void **objects;
> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index 571cf0d6ec01..7015edce5bd8 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -356,7 +356,10 @@ static inline void *mmu_memory_cache_alloc_obj(struct kvm_mmu_memory_cache *mc,
>  	if (mc->kmem_cache)
>  		return kmem_cache_alloc(mc->kmem_cache, gfp_flags);
>  
> -	page = (void *)__get_free_page(gfp_flags);
> +	if (mc->page_get)
> +		page = (void *)mc->page_get(gfp_flags);
> +	else
> +		page = (void *)__get_free_page(gfp_flags);
>  	if (page && mc->init_value)
>  		memset64(page, mc->init_value, PAGE_SIZE / sizeof(u64));
>  	return page;
> @@ -416,6 +419,8 @@ void kvm_mmu_free_memory_cache(struct kvm_mmu_memory_cache *mc)
>  	while (mc->nobjs) {
>  		if (mc->kmem_cache)
>  			kmem_cache_free(mc->kmem_cache, mc->objects[--mc->nobjs]);
> +		else if (mc->page_free)
> +			mc->page_free((unsigned long)mc->objects[--mc->nobjs]);
>  		else
>  			free_page((unsigned long)mc->objects[--mc->nobjs]);
>  	}
> -- 
> 2.53.0.rc1.217.geba53bf80e-goog