Re: [PATCH v17 14/24] KVM: x86/mmu: Enforce guest_memfd's max order when recovering hugepages

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

On 7/30/2025 6:54 AM, Sean Christopherson wrote:
> Rework kvm_mmu_max_mapping_level() to provide the plumbing to consult
> guest_memfd (and relevant vendor code) when recovering hugepages, e.g.
> after disabling live migration.  The flaw has existed since guest_memfd was
> originally added, but has gone unnoticed due to lack of guest_memfd support
> for hugepages or dirty logging.
> 
> Don't actually call into guest_memfd at this time, as it's unclear as to
> what the API should be.  Ideally, KVM would simply use kvm_gmem_get_pfn(),
> but invoking kvm_gmem_get_pfn() would lead to sleeping in atomic context
> if guest_memfd needed to allocate memory (mmu_lock is held).  Luckily,
> the path isn't actually reachable, so just add a TODO and WARN to ensure
> the functionality is added alongisde guest_memfd hugepage support, and
> punt the guest_memfd API design question to the future.
> 
> Note, calling kvm_mem_is_private() in the non-fault path is safe, so long
> as mmu_lock is held, as hugepage recovery operates on shadow-present SPTEs,
> i.e. calling kvm_mmu_max_mapping_level() with @fault=NULL is mutually
> exclusive with kvm_vm_set_mem_attributes() changing the PRIVATE attribute
> of the gfn.
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>   arch/x86/kvm/mmu/mmu.c          | 82 +++++++++++++++++++--------------
>   arch/x86/kvm/mmu/mmu_internal.h |  2 +-
>   arch/x86/kvm/mmu/tdp_mmu.c      |  2 +-
>   3 files changed, 49 insertions(+), 37 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 20dd9f64156e..61eb9f723675 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3302,31 +3302,54 @@ static u8 kvm_max_level_for_order(int order)
>   	return PG_LEVEL_4K;
>   }
>   
> -static u8 kvm_max_private_mapping_level(struct kvm *kvm, kvm_pfn_t pfn,
> -					u8 max_level, int gmem_order)
> +static u8 kvm_max_private_mapping_level(struct kvm *kvm, struct kvm_page_fault *fault,
> +					const struct kvm_memory_slot *slot, gfn_t gfn)

I don't see why slot and gfn are needed here. Just to keep consistent 
with host_pfn_mapping_level()?

>   {
> -	u8 req_max_level;
> +	u8 max_level, coco_level;
> +	kvm_pfn_t pfn;
>   
> -	if (max_level == PG_LEVEL_4K)
> -		return PG_LEVEL_4K;
> +	/* For faults, use the gmem information that was resolved earlier. */
> +	if (fault) {
> +		pfn = fault->pfn;
> +		max_level = fault->max_level;
> +	} else {
> +		/* TODO: Call into guest_memfd once hugepages are supported. */
> +		WARN_ONCE(1, "Get pfn+order from guest_memfd");
> +		pfn = KVM_PFN_ERR_FAULT;
> +		max_level = PG_LEVEL_4K;
> +	}
>   
> -	max_level = min(kvm_max_level_for_order(gmem_order), max_level);
>   	if (max_level == PG_LEVEL_4K)
> -		return PG_LEVEL_4K;
> +		return max_level;
>   
> -	req_max_level = kvm_x86_call(gmem_max_mapping_level)(kvm, pfn);
> -	if (req_max_level)
> -		max_level = min(max_level, req_max_level);
> +	/*
> +	 * CoCo may influence the max mapping level, e.g. due to RMP or S-EPT
> +	 * restrictions.  A return of '0' means "no additional restrictions", to
> +	 * allow for using an optional "ret0" static call.
> +	 */
> +	coco_level = kvm_x86_call(gmem_max_mapping_level)(kvm, pfn);
> +	if (coco_level)
> +		max_level = min(max_level, coco_level);
>   
>   	return max_level;
>   }
>   
> -static int __kvm_mmu_max_mapping_level(struct kvm *kvm,
> -				       const struct kvm_memory_slot *slot,
> -				       gfn_t gfn, int max_level, bool is_private)
> +int kvm_mmu_max_mapping_level(struct kvm *kvm, struct kvm_page_fault *fault,
> +			      const struct kvm_memory_slot *slot, gfn_t gfn)
>   {
>   	struct kvm_lpage_info *linfo;
> -	int host_level;
> +	int host_level, max_level;
> +	bool is_private;
> +
> +	lockdep_assert_held(&kvm->mmu_lock);
> +
> +	if (fault) {
> +		max_level = fault->max_level;
> +		is_private = fault->is_private;
> +	} else {
> +		max_level = PG_LEVEL_NUM;
> +		is_private = kvm_mem_is_private(kvm, gfn);
> +	}
>   
>   	max_level = min(max_level, max_huge_page_level);
>   	for ( ; max_level > PG_LEVEL_4K; max_level--) {
> @@ -3335,25 +3358,16 @@ static int __kvm_mmu_max_mapping_level(struct kvm *kvm,
>   			break;
>   	}
>   
> +	if (max_level == PG_LEVEL_4K)
> +		return PG_LEVEL_4K;
> +
>   	if (is_private)
> -		return max_level;
> -
> -	if (max_level == PG_LEVEL_4K)
> -		return PG_LEVEL_4K;
> -
> -	host_level = host_pfn_mapping_level(kvm, gfn, slot);
> +		host_level = kvm_max_private_mapping_level(kvm, fault, slot, gfn);
> +	else
> +		host_level = host_pfn_mapping_level(kvm, gfn, slot);
>   	return min(host_level, max_level);
>   }
>   
> -int kvm_mmu_max_mapping_level(struct kvm *kvm,
> -			      const struct kvm_memory_slot *slot, gfn_t gfn)
> -{
> -	bool is_private = kvm_slot_has_gmem(slot) &&
> -			  kvm_mem_is_private(kvm, gfn);
> -
> -	return __kvm_mmu_max_mapping_level(kvm, slot, gfn, PG_LEVEL_NUM, is_private);
> -}
> -
>   void kvm_mmu_hugepage_adjust(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>   {
>   	struct kvm_memory_slot *slot = fault->slot;
> @@ -3374,9 +3388,8 @@ void kvm_mmu_hugepage_adjust(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
>   	 * Enforce the iTLB multihit workaround after capturing the requested
>   	 * level, which will be used to do precise, accurate accounting.
>   	 */
> -	fault->req_level = __kvm_mmu_max_mapping_level(vcpu->kvm, slot,
> -						       fault->gfn, fault->max_level,
> -						       fault->is_private);
> +	fault->req_level = kvm_mmu_max_mapping_level(vcpu->kvm, fault,
> +						     fault->slot, fault->gfn);
>   	if (fault->req_level == PG_LEVEL_4K || fault->huge_page_disallowed)
>   		return;
>   
> @@ -4564,8 +4577,7 @@ static int kvm_mmu_faultin_pfn_private(struct kvm_vcpu *vcpu,
>   	}
>   
>   	fault->map_writable = !(fault->slot->flags & KVM_MEM_READONLY);
> -	fault->max_level = kvm_max_private_mapping_level(vcpu->kvm, fault->pfn,
> -							 fault->max_level, max_order);
> +	fault->max_level = kvm_max_level_for_order(max_order);
>   
>   	return RET_PF_CONTINUE;
>   }
> @@ -7165,7 +7177,7 @@ static bool kvm_mmu_zap_collapsible_spte(struct kvm *kvm,
>   		 * mapping if the indirect sp has level = 1.
>   		 */
>   		if (sp->role.direct &&
> -		    sp->role.level < kvm_mmu_max_mapping_level(kvm, slot, sp->gfn)) {
> +		    sp->role.level < kvm_mmu_max_mapping_level(kvm, NULL, slot, sp->gfn)) {
>   			kvm_zap_one_rmap_spte(kvm, rmap_head, sptep);
>   
>   			if (kvm_available_flush_remote_tlbs_range())
> diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
> index 65f3c89d7c5d..b776be783a2f 100644
> --- a/arch/x86/kvm/mmu/mmu_internal.h
> +++ b/arch/x86/kvm/mmu/mmu_internal.h
> @@ -411,7 +411,7 @@ static inline int kvm_mmu_do_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa,
>   	return r;
>   }
>   
> -int kvm_mmu_max_mapping_level(struct kvm *kvm,
> +int kvm_mmu_max_mapping_level(struct kvm *kvm, struct kvm_page_fault *fault,
>   			      const struct kvm_memory_slot *slot, gfn_t gfn);
>   void kvm_mmu_hugepage_adjust(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault);
>   void disallowed_hugepage_adjust(struct kvm_page_fault *fault, u64 spte, int cur_level);
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 7f3d7229b2c1..740cb06accdb 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1813,7 +1813,7 @@ static void recover_huge_pages_range(struct kvm *kvm,
>   		if (iter.gfn < start || iter.gfn >= end)
>   			continue;
>   
> -		max_mapping_level = kvm_mmu_max_mapping_level(kvm, slot, iter.gfn);
> +		max_mapping_level = kvm_mmu_max_mapping_level(kvm, NULL, slot, iter.gfn);
>   		if (max_mapping_level < iter.level)
>   			continue;
>