Re: [PATCH v6 8/9] KVM: guest_memfd: Explicitly pass number of pages to make_private() hook

[Xiaoyao Li <xiaoyao.li@intel.com>]

On 7/24/2026 5:08 AM, Sean Christopherson wrote:
> Tweak the guest_memfd make_private() hook to explicitly pass the number of
> pages to align with the signature of the make_shared() hook, and because
> the existing code is outright broken if a guest_memfd folio is comprised of
> more than one page (which can't happen, yet).  The SNP code *tries* to
> create a corresponding huge entry, but if the RMP must use 4KiB entries for
> whatever reason, KVM will only convert the first pfn, and not the entire
> range of pfns that will be mapped into the guest.
> 
> Alternatively, @max_order could simply be repurposed as _the_ @order, but
> that will fall apart when in-place conversion comes along, at which point
> KVM will need to deal with conversions that aren't bound 1:1 to a folio.
> I.e. the number of pages to convert may not be exactly be a power-of-2 (and

one redundant 'be'

> folios don't strictly guarantee power-of-2 pages anyways).
> 
> WARN in the SNP code if the number of pages to prepare is anything other
> than '1', i.e. if guest_memfd is trying to prepare/convert more than a
> single 4KiB page, as sev_gmem_prepare() doesn't actually handle conversion
> greater than order-0 folios.
> 
> Opportunistically swap the ordering of @pfn and @gfn params for
> kvm_x86_ops.gmem_make_private() to match kvm_arch_gmem_make_private().
> 
> Fixes: b85524314a3d ("KVM: guest_memfd: delay kvm_gmem_prepare_folio() until the memory is passed to the guest")
> Signed-off-by: Sean Christopherson <seanjc@google.com>

Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>

> ---
>   arch/x86/include/asm/kvm_host.h |  3 ++-
>   arch/x86/kvm/svm/sev.c          | 27 +++++++++++----------------
>   arch/x86/kvm/svm/svm.h          |  2 +-
>   arch/x86/kvm/x86.c              |  5 +++--
>   include/linux/kvm_host.h        |  2 +-
>   virt/kvm/guest_memfd.c          |  2 +-
>   6 files changed, 19 insertions(+), 22 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 4be57157136b..230267b2203b 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1725,7 +1725,8 @@ struct kvm_x86_ops {
>   	gva_t (*get_untagged_addr)(struct kvm_vcpu *vcpu, gva_t gva, unsigned int flags);
>   	void *(*alloc_apic_backing_page)(struct kvm_vcpu *vcpu);
>   #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT
> -	int (*gmem_make_private)(struct kvm *kvm, kvm_pfn_t pfn, gfn_t gfn, int max_order);
> +	int (*gmem_make_private)(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
> +				 kvm_pfn_t nr_pages);
>   #endif
>   #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_RECLAIM
>   	void (*gmem_make_shared)(kvm_pfn_t pfn, kvm_pfn_t nr_pages);
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 0ecba768c153..bf7f94d1d7f9 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -5090,15 +5090,7 @@ static bool is_pfn_range_shared(kvm_pfn_t start, kvm_pfn_t end)
>   	return true;
>   }
>   
> -static u8 max_level_for_order(int order)
> -{
> -	if (order >= KVM_HPAGE_GFN_SHIFT(PG_LEVEL_2M))
> -		return PG_LEVEL_2M;
> -
> -	return PG_LEVEL_4K;
> -}
> -
> -static bool is_large_rmp_possible(struct kvm *kvm, kvm_pfn_t pfn, int order)
> +static bool is_large_rmp_possible(kvm_pfn_t pfn, kvm_pfn_t nr_pages)
>   {
>   	kvm_pfn_t pfn_aligned = ALIGN_DOWN(pfn, PTRS_PER_PMD);
>   
> @@ -5107,14 +5099,14 @@ static bool is_large_rmp_possible(struct kvm *kvm, kvm_pfn_t pfn, int order)
>   	 * PFN is currently shared, then the entire 2M-aligned range can be
>   	 * set to private via a single 2M RMP entry.
>   	 */
> -	if (max_level_for_order(order) > PG_LEVEL_4K &&
> +	if (nr_pages >= KVM_PAGES_PER_HPAGE(PG_LEVEL_2M) &&
>   	    is_pfn_range_shared(pfn_aligned, pfn_aligned + PTRS_PER_PMD))
>   		return true;
>   
>   	return false;
>   }
>   
> -int sev_gmem_make_private(struct kvm *kvm, kvm_pfn_t pfn, gfn_t gfn, int max_order)
> +int sev_gmem_make_private(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn, kvm_pfn_t nr_pages)
>   {
>   	struct kvm_sev_info *sev = to_kvm_sev_info(kvm);
>   	kvm_pfn_t pfn_aligned;
> @@ -5125,6 +5117,9 @@ int sev_gmem_make_private(struct kvm *kvm, kvm_pfn_t pfn, gfn_t gfn, int max_ord
>   	if (!sev_snp_guest(kvm))
>   		return 0;
>   
> +	if (WARN_ON_ONCE(nr_pages != 1))
> +		return -EIO;
> +
>   	rc = snp_lookup_rmpentry(pfn, &assigned, &level);
>   	if (rc) {
>   		pr_err_ratelimited("SEV: Failed to look up RMP entry: GFN %llx PFN %llx error %d\n",
> @@ -5133,12 +5128,12 @@ int sev_gmem_make_private(struct kvm *kvm, kvm_pfn_t pfn, gfn_t gfn, int max_ord
>   	}
>   
>   	if (assigned) {
> -		pr_debug("%s: already assigned: gfn %llx pfn %llx max_order %d level %d\n",
> -			 __func__, gfn, pfn, max_order, level);
> +		pr_debug("%s: already assigned: gfn %llx pfn %llx nr_pages %llx level %d\n",
> +			 __func__, gfn, pfn, nr_pages, level);
>   		return 0;
>   	}
>   
> -	if (is_large_rmp_possible(kvm, pfn, max_order)) {
> +	if (is_large_rmp_possible(pfn, nr_pages)) {
>   		level = PG_LEVEL_2M;
>   		pfn_aligned = ALIGN_DOWN(pfn, PTRS_PER_PMD);
>   		gfn_aligned = ALIGN_DOWN(gfn, PTRS_PER_PMD);
> @@ -5155,8 +5150,8 @@ int sev_gmem_make_private(struct kvm *kvm, kvm_pfn_t pfn, gfn_t gfn, int max_ord
>   		return -EINVAL;
>   	}
>   
> -	pr_debug("%s: updated: gfn %llx pfn %llx pfn_aligned %llx max_order %d level %d\n",
> -		 __func__, gfn, pfn, pfn_aligned, max_order, level);
> +	pr_debug("%s: updated: gfn %llx pfn %llx pfn_aligned %llx nr_pages %llx level %d\n",
> +		 __func__, gfn, pfn, pfn_aligned, nr_pages, level);
>   
>   	return 0;
>   }
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index ad849edcacc7..a327bf751ecd 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -1009,7 +1009,7 @@ int sev_cpu_init(struct svm_cpu_data *sd);
>   int sev_dev_get_attr(u32 group, u64 attr, u64 *val);
>   extern unsigned int max_sev_asid;
>   void sev_handle_rmp_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code);
> -int sev_gmem_make_private(struct kvm *kvm, kvm_pfn_t pfn, gfn_t gfn, int max_order);
> +int sev_gmem_make_private(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn, kvm_pfn_t nr_pages);
>   void sev_gmem_make_shared(kvm_pfn_t pfn, kvm_pfn_t nr_pages);
>   void sev_gmem_invalidate_range(struct kvm *kvm, struct kvm_gfn_range *range);
>   int sev_gmem_max_mapping_level(struct kvm *kvm, kvm_pfn_t pfn, bool is_private);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index e7865b28d37b..511830d1b6f3 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -10620,9 +10620,10 @@ bool kvm_arch_supports_gmem_init_shared(struct kvm *kvm)
>   }
>   
>   #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT
> -int kvm_arch_gmem_make_private(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn, int max_order)
> +int kvm_arch_gmem_make_private(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
> +			       kvm_pfn_t nr_pages)
>   {
> -	return kvm_x86_call(gmem_make_private)(kvm, pfn, gfn, max_order);
> +	return kvm_x86_call(gmem_make_private)(kvm, gfn, pfn, nr_pages);
>   }
>   #endif
>   
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 9cf01429281b..97cedf8b89c5 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -2581,7 +2581,7 @@ static inline int kvm_gmem_get_pfn(struct kvm *kvm,
>   
>   #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT
>   int kvm_arch_gmem_make_private(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
> -			       int max_order);
> +			       kvm_pfn_t nr_pages);
>   #endif
>   
>   #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_POPULATE
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index 466f611ad4b7..255860c472ee 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -77,7 +77,7 @@ static int __kvm_gmem_prepare_folio(struct kvm *kvm, struct kvm_memory_slot *slo
>   	kvm_pfn_t pfn = folio_file_pfn(folio, index);
>   	gfn_t gfn = slot->base_gfn + index - slot->gmem.pgoff;
>   
> -	return kvm_arch_gmem_make_private(kvm, gfn, pfn, folio_order(folio));
> +	return kvm_arch_gmem_make_private(kvm, gfn, pfn, folio_nr_pages(folio));
>   #else
>   	return 0;
>   #endif