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Message-ID: <CA+EHjTxHn-wFH08Z3sV+dq3aGGGmwtinD1qePABSiG6f5xW+WA@mail.gmail.com>
Date: Thu, 31 Jul 2025 09:10:00 +0100
From: Fuad Tabba <tabba@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: Paolo Bonzini <pbonzini@...hat.com>, Marc Zyngier <maz@...nel.org>,
Oliver Upton <oliver.upton@...ux.dev>, kvm@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.linux.dev,
linux-kernel@...r.kernel.org, Ira Weiny <ira.weiny@...el.com>,
Gavin Shan <gshan@...hat.com>, Shivank Garg <shivankg@....com>, Vlastimil Babka <vbabka@...e.cz>,
Xiaoyao Li <xiaoyao.li@...el.com>, David Hildenbrand <david@...hat.com>,
Ackerley Tng <ackerleytng@...gle.com>, Tao Chan <chentao@...inos.cn>,
James Houghton <jthoughton@...gle.com>
Subject: Re: [PATCH v17 14/24] KVM: x86/mmu: Enforce guest_memfd's max order
when recovering hugepages
On Tue, 29 Jul 2025 at 23:55, Sean Christopherson <seanjc@...gle.com> 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.
nit: *alongside
>
> 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@...gle.com>
> ---
Reviewed-by: Fuad Tabba <tabba@...gle.com>
Cheers,
/fuad
> 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)
> {
> - 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;
>
> --
> 2.50.1.552.g942d659e1b-goog
>
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