Re: [PATCH v13 20/35] KVM: x86/mmu: Handle page fault for private memory

[Fuad Tabba <tabba@google.com>]

On Fri, Oct 27, 2023 at 7:23 PM Sean Christopherson <seanjc@google.com> wrote:
>
> From: Chao Peng <chao.p.peng@linux.intel.com>
>
> Add support for resolving page faults on guest private memory for VMs
> that differentiate between "shared" and "private" memory.  For such VMs,
> KVM_MEM_PRIVATE memslots can include both fd-based private memory and
> hva-based shared memory, and KVM needs to map in the "correct" variant,
> i.e. KVM needs to map the gfn shared/private as appropriate based on the
> current state of the gfn's KVM_MEMORY_ATTRIBUTE_PRIVATE flag.
>
> For AMD's SEV-SNP and Intel's TDX, the guest effectively gets to request
> shared vs. private via a bit in the guest page tables, i.e. what the guest
> wants may conflict with the current memory attributes.  To support such
> "implicit" conversion requests, exit to user with KVM_EXIT_MEMORY_FAULT
> to forward the request to userspace.  Add a new flag for memory faults,
> KVM_MEMORY_EXIT_FLAG_PRIVATE, to communicate whether the guest wants to
> map memory as shared vs. private.
>
> Like KVM_MEMORY_ATTRIBUTE_PRIVATE, use bit 3 for flagging private memory
> so that KVM can use bits 0-2 for capturing RWX behavior if/when userspace
> needs such information, e.g. a likely user of KVM_EXIT_MEMORY_FAULT is to
> exit on missing mappings when handling guest page fault VM-Exits.  In
> that case, userspace will want to know RWX information in order to
> correctly/precisely resolve the fault.
>
> Note, private memory *must* be backed by guest_memfd, i.e. shared mappings
> always come from the host userspace page tables, and private mappings
> always come from a guest_memfd instance.
>
> Co-developed-by: Yu Zhang <yu.c.zhang@linux.intel.com>
> Signed-off-by: Yu Zhang <yu.c.zhang@linux.intel.com>
> Signed-off-by: Chao Peng <chao.p.peng@linux.intel.com>
> Co-developed-by: Sean Christopherson <seanjc@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---

With my limited understanding of kvm-x86 mmu code:
Reviewed-by: Fuad Tabba <tabba@google.com>

Tested the x86 code (as part of this patch series) on qemu. The x86
fault handling code was used as a guide to how it should be done for
pKVM/arm64 (with similar code added there):
Tested-by: Fuad Tabba <tabba@google.com>

Cheers,
/fuad

>  Documentation/virt/kvm/api.rst  |   8 ++-
>  arch/x86/kvm/mmu/mmu.c          | 101 ++++++++++++++++++++++++++++++--
>  arch/x86/kvm/mmu/mmu_internal.h |   1 +
>  include/linux/kvm_host.h        |   8 ++-
>  include/uapi/linux/kvm.h        |   1 +
>  5 files changed, 110 insertions(+), 9 deletions(-)
>
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 7f00c310c24a..38dc1fda4f45 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -6837,6 +6837,7 @@ spec refer, https://github.com/riscv/riscv-sbi-doc.
>
>                 /* KVM_EXIT_MEMORY_FAULT */
>                 struct {
> +  #define KVM_MEMORY_EXIT_FLAG_PRIVATE (1ULL << 3)
>                         __u64 flags;
>                         __u64 gpa;
>                         __u64 size;
> @@ -6845,8 +6846,11 @@ spec refer, https://github.com/riscv/riscv-sbi-doc.
>  KVM_EXIT_MEMORY_FAULT indicates the vCPU has encountered a memory fault that
>  could not be resolved by KVM.  The 'gpa' and 'size' (in bytes) describe the
>  guest physical address range [gpa, gpa + size) of the fault.  The 'flags' field
> -describes properties of the faulting access that are likely pertinent.
> -Currently, no flags are defined.
> +describes properties of the faulting access that are likely pertinent:
> +
> + - KVM_MEMORY_EXIT_FLAG_PRIVATE - When set, indicates the memory fault occurred
> +   on a private memory access.  When clear, indicates the fault occurred on a
> +   shared access.
>
>  Note!  KVM_EXIT_MEMORY_FAULT is unique among all KVM exit reasons in that it
>  accompanies a return code of '-1', not '0'!  errno will always be set to EFAULT
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 4167d557c577..c4e758f0aebb 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3147,9 +3147,9 @@ static int host_pfn_mapping_level(struct kvm *kvm, gfn_t gfn,
>         return level;
>  }
>
> -int kvm_mmu_max_mapping_level(struct kvm *kvm,
> -                             const struct kvm_memory_slot *slot, gfn_t gfn,
> -                             int 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)
>  {
>         struct kvm_lpage_info *linfo;
>         int host_level;
> @@ -3161,6 +3161,9 @@ int kvm_mmu_max_mapping_level(struct kvm *kvm,
>                         break;
>         }
>
> +       if (is_private)
> +               return max_level;
> +
>         if (max_level == PG_LEVEL_4K)
>                 return PG_LEVEL_4K;
>
> @@ -3168,6 +3171,16 @@ int kvm_mmu_max_mapping_level(struct kvm *kvm,
>         return min(host_level, max_level);
>  }
>
> +int kvm_mmu_max_mapping_level(struct kvm *kvm,
> +                             const struct kvm_memory_slot *slot, gfn_t gfn,
> +                             int max_level)
> +{
> +       bool is_private = kvm_slot_can_be_private(slot) &&
> +                         kvm_mem_is_private(kvm, gfn);
> +
> +       return __kvm_mmu_max_mapping_level(kvm, slot, gfn, max_level, is_private);
> +}
> +
>  void kvm_mmu_hugepage_adjust(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>  {
>         struct kvm_memory_slot *slot = fault->slot;
> @@ -3188,8 +3201,9 @@ 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->req_level = __kvm_mmu_max_mapping_level(vcpu->kvm, slot,
> +                                                      fault->gfn, fault->max_level,
> +                                                      fault->is_private);
>         if (fault->req_level == PG_LEVEL_4K || fault->huge_page_disallowed)
>                 return;
>
> @@ -4261,6 +4275,55 @@ void kvm_arch_async_page_ready(struct kvm_vcpu *vcpu, struct kvm_async_pf *work)
>         kvm_mmu_do_page_fault(vcpu, work->cr2_or_gpa, 0, true, NULL);
>  }
>
> +static inline u8 kvm_max_level_for_order(int order)
> +{
> +       BUILD_BUG_ON(KVM_MAX_HUGEPAGE_LEVEL > PG_LEVEL_1G);
> +
> +       KVM_MMU_WARN_ON(order != KVM_HPAGE_GFN_SHIFT(PG_LEVEL_1G) &&
> +                       order != KVM_HPAGE_GFN_SHIFT(PG_LEVEL_2M) &&
> +                       order != KVM_HPAGE_GFN_SHIFT(PG_LEVEL_4K));
> +
> +       if (order >= KVM_HPAGE_GFN_SHIFT(PG_LEVEL_1G))
> +               return PG_LEVEL_1G;
> +
> +       if (order >= KVM_HPAGE_GFN_SHIFT(PG_LEVEL_2M))
> +               return PG_LEVEL_2M;
> +
> +       return PG_LEVEL_4K;
> +}
> +
> +static void kvm_mmu_prepare_memory_fault_exit(struct kvm_vcpu *vcpu,
> +                                             struct kvm_page_fault *fault)
> +{
> +       kvm_prepare_memory_fault_exit(vcpu, fault->gfn << PAGE_SHIFT,
> +                                     PAGE_SIZE, fault->write, fault->exec,
> +                                     fault->is_private);
> +}
> +
> +static int kvm_faultin_pfn_private(struct kvm_vcpu *vcpu,
> +                                  struct kvm_page_fault *fault)
> +{
> +       int max_order, r;
> +
> +       if (!kvm_slot_can_be_private(fault->slot)) {
> +               kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
> +               return -EFAULT;
> +       }
> +
> +       r = kvm_gmem_get_pfn(vcpu->kvm, fault->slot, fault->gfn, &fault->pfn,
> +                            &max_order);
> +       if (r) {
> +               kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
> +               return r;
> +       }
> +
> +       fault->max_level = min(kvm_max_level_for_order(max_order),
> +                              fault->max_level);
> +       fault->map_writable = !(fault->slot->flags & KVM_MEM_READONLY);
> +
> +       return RET_PF_CONTINUE;
> +}
> +
>  static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
>  {
>         struct kvm_memory_slot *slot = fault->slot;
> @@ -4293,6 +4356,14 @@ static int __kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
>                         return RET_PF_EMULATE;
>         }
>
> +       if (fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)) {
> +               kvm_mmu_prepare_memory_fault_exit(vcpu, fault);
> +               return -EFAULT;
> +       }
> +
> +       if (fault->is_private)
> +               return kvm_faultin_pfn_private(vcpu, fault);
> +
>         async = false;
>         fault->pfn = __gfn_to_pfn_memslot(slot, fault->gfn, false, false, &async,
>                                           fault->write, &fault->map_writable,
> @@ -7173,6 +7244,26 @@ void kvm_mmu_pre_destroy_vm(struct kvm *kvm)
>  }
>
>  #ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
> +bool kvm_arch_pre_set_memory_attributes(struct kvm *kvm,
> +                                       struct kvm_gfn_range *range)
> +{
> +       /*
> +        * Zap SPTEs even if the slot can't be mapped PRIVATE.  KVM x86 only
> +        * supports KVM_MEMORY_ATTRIBUTE_PRIVATE, and so it *seems* like KVM
> +        * can simply ignore such slots.  But if userspace is making memory
> +        * PRIVATE, then KVM must prevent the guest from accessing the memory
> +        * as shared.  And if userspace is making memory SHARED and this point
> +        * is reached, then at least one page within the range was previously
> +        * PRIVATE, i.e. the slot's possible hugepage ranges are changing.
> +        * Zapping SPTEs in this case ensures KVM will reassess whether or not
> +        * a hugepage can be used for affected ranges.
> +        */
> +       if (WARN_ON_ONCE(!kvm_arch_has_private_mem(kvm)))
> +               return false;
> +
> +       return kvm_unmap_gfn_range(kvm, range);
> +}
> +
>  static bool hugepage_test_mixed(struct kvm_memory_slot *slot, gfn_t gfn,
>                                 int level)
>  {
> diff --git a/arch/x86/kvm/mmu/mmu_internal.h b/arch/x86/kvm/mmu/mmu_internal.h
> index decc1f153669..86c7cb692786 100644
> --- a/arch/x86/kvm/mmu/mmu_internal.h
> +++ b/arch/x86/kvm/mmu/mmu_internal.h
> @@ -201,6 +201,7 @@ struct kvm_page_fault {
>
>         /* Derived from mmu and global state.  */
>         const bool is_tdp;
> +       const bool is_private;
>         const bool nx_huge_page_workaround_enabled;
>
>         /*
> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
> index 7de93858054d..e3223cafd7db 100644
> --- a/include/linux/kvm_host.h
> +++ b/include/linux/kvm_host.h
> @@ -2358,14 +2358,18 @@ static inline void kvm_account_pgtable_pages(void *virt, int nr)
>  #define  KVM_DIRTY_RING_MAX_ENTRIES  65536
>
>  static inline void kvm_prepare_memory_fault_exit(struct kvm_vcpu *vcpu,
> -                                                gpa_t gpa, gpa_t size)
> +                                                gpa_t gpa, gpa_t size,
> +                                                bool is_write, bool is_exec,
> +                                                bool is_private)
>  {
>         vcpu->run->exit_reason = KVM_EXIT_MEMORY_FAULT;
>         vcpu->run->memory_fault.gpa = gpa;
>         vcpu->run->memory_fault.size = size;
>
> -       /* Flags are not (yet) defined or communicated to userspace. */
> +       /* RWX flags are not (yet) defined or communicated to userspace. */
>         vcpu->run->memory_fault.flags = 0;
> +       if (is_private)
> +               vcpu->run->memory_fault.flags |= KVM_MEMORY_EXIT_FLAG_PRIVATE;
>  }
>
>  #ifdef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES
> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
> index 33d542de0a61..29e9eb51dec9 100644
> --- a/include/uapi/linux/kvm.h
> +++ b/include/uapi/linux/kvm.h
> @@ -527,6 +527,7 @@ struct kvm_run {
>                 } notify;
>                 /* KVM_EXIT_MEMORY_FAULT */
>                 struct {
> +#define KVM_MEMORY_EXIT_FLAG_PRIVATE   (1ULL << 3)
>                         __u64 flags;
>                         __u64 gpa;
>                         __u64 size;
> --
> 2.42.0.820.g83a721a137-goog
>