Re: [PATCH v4 12/16] x86/virt/tdx: Add helpers to allow for pre-allocating pages

[Binbin Wu]

On 11/21/2025 8:51 AM, Rick Edgecombe wrote:
> In the KVM fault path page, tables and private pages need to be
"In the KVM fault path page, tables ..." should be
"In the KVM fault path, page tables ..."

> installed under a spin lock. This means that the operations around
> installing PAMT pages for them will not be able to allocate pages.
[...]
> @@ -141,7 +142,46 @@ int tdx_guest_keyid_alloc(void);
>   u32 tdx_get_nr_guest_keyids(void);
>   void tdx_guest_keyid_free(unsigned int keyid);
>   
> -int tdx_pamt_get(struct page *page);
> +int tdx_dpamt_entry_pages(void);
> +
> +/*
> + * Simple structure for pre-allocating Dynamic
> + * PAMT pages outside of locks.

It's not just for Dynamic PAMT pages, but also external page table pages.

> + */
> +struct tdx_prealloc {
> +	struct list_head page_list;
> +	int cnt;
> +};
> +
> +static inline struct page *get_tdx_prealloc_page(struct tdx_prealloc *prealloc)
> +{
> +	struct page *page;
> +
> +	page = list_first_entry_or_null(&prealloc->page_list, struct page, lru);
> +	if (page) {
> +		list_del(&page->lru);
> +		prealloc->cnt--;
> +	}
> +
> +	return page;
> +}
> +
> +static inline int topup_tdx_prealloc_page(struct tdx_prealloc *prealloc, unsigned int min_size)
> +{
> +	while (prealloc->cnt < min_size) {
> +		struct page *page = alloc_page(GFP_KERNEL_ACCOUNT);
> +
> +		if (!page)
> +			return -ENOMEM;
> +
> +		list_add(&page->lru, &prealloc->page_list);
> +		prealloc->cnt++;
> +	}
> +
> +	return 0;
> +}
> +
> +int tdx_pamt_get(struct page *page, struct tdx_prealloc *prealloc);
>   void tdx_pamt_put(struct page *page);
>   
>   struct page *tdx_alloc_page(void);
> @@ -219,6 +259,7 @@ static inline int tdx_enable(void)  { return -ENODEV; }
>   static inline u32 tdx_get_nr_guest_keyids(void) { return 0; }
>   static inline const char *tdx_dump_mce_info(struct mce *m) { return NULL; }
>   static inline const struct tdx_sys_info *tdx_get_sysinfo(void) { return NULL; }
> +static inline int tdx_dpamt_entry_pages(void) { return 0; }
>   #endif	/* CONFIG_INTEL_TDX_HOST */
>   
>   #ifdef CONFIG_KEXEC_CORE
> diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> index 260bb0e6eb44..61a058a8f159 100644
> --- a/arch/x86/kvm/vmx/tdx.c
> +++ b/arch/x86/kvm/vmx/tdx.c
> @@ -1644,23 +1644,34 @@ static int tdx_mem_page_add(struct kvm *kvm, gfn_t gfn, enum pg_level level,
>   
>   static void *tdx_alloc_external_fault_cache(struct kvm_vcpu *vcpu)
>   {
> -	struct vcpu_tdx *tdx = to_tdx(vcpu);
> +	struct page *page = get_tdx_prealloc_page(&to_tdx(vcpu)->prealloc);
>   
> -	return kvm_mmu_memory_cache_alloc(&tdx->mmu_external_spt_cache);
> +	if (WARN_ON_ONCE(!page))
> +		return (void *)__get_free_page(GFP_ATOMIC | __GFP_ACCOUNT);

kvm_mmu_memory_cache_alloc() calls BUG_ON() if the atomic allocation failed.
Do we want to follow?

> +
> +	return page_address(page);
>   }
>   
>   static int tdx_topup_external_fault_cache(struct kvm_vcpu *vcpu, unsigned int cnt)
>   {
> -	struct vcpu_tdx *tdx = to_tdx(vcpu);
> +	struct tdx_prealloc *prealloc = &to_tdx(vcpu)->prealloc;
> +	int min_fault_cache_size;
>   
> -	return kvm_mmu_topup_memory_cache(&tdx->mmu_external_spt_cache, cnt);
> +	/* External page tables */
> +	min_fault_cache_size = cnt;
> +	/* Dynamic PAMT pages (if enabled) */
> +	min_fault_cache_size += tdx_dpamt_entry_pages() * PT64_ROOT_MAX_LEVEL;

Is the value PT64_ROOT_MAX_LEVEL intended, since dynamic PAMT pages are only
needed for 4KB level?

> +
> +	return topup_tdx_prealloc_page(prealloc, min_fault_cache_size);
>   }
>   
[...]