Re: [PATCH v9 7/8] KVM: Handle page fault for private memory

Ackerley Tng
2022-11-16T20:50:38Z

> A memslot with KVM_MEM_PRIVATE being set can include both fd-based
> private memory and hva-based shared memory. Architecture code (like TDX
> code) can tell whether the on-going fault is private or not. This patch
> adds a 'is_private' field to kvm_page_fault to indicate this and
> architecture code is expected to set it.
>
> To handle page fault for such memslot, the handling logic is different
> depending on whether the fault is private or shared. KVM checks if
> 'is_private' matches the host's view of the page (maintained in
> mem_attr_array).
> - For a successful match, private pfn is obtained with
> restrictedmem_get_page () from private fd and shared pfn is obtained
> with existing get_user_pages().
> - For a failed match, KVM causes a KVM_EXIT_MEMORY_FAULT exit to
> userspace. Userspace then can convert memory between private/shared
> in host's view and retry the fault.
>
> 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>
> ---
> arch/x86/kvm/mmu/mmu.c | 56 +++++++++++++++++++++++++++++++--
> arch/x86/kvm/mmu/mmu_internal.h | 14 ++++++++-
> arch/x86/kvm/mmu/mmutrace.h | 1 +
> arch/x86/kvm/mmu/spte.h | 6 ++++
> arch/x86/kvm/mmu/tdp_mmu.c | 3 +-
> include/linux/kvm_host.h | 28 +++++++++++++++++
> 6 files changed, 103 insertions(+), 5 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu/mmu.c b/arch/x86/kvm/mmu/mmu.c
> index 67a9823a8c35..10017a9f26ee 100644
> --- a/arch/x86/kvm/mmu/mmu.c
> +++ b/arch/x86/kvm/mmu/mmu.c
> @@ -3030,7 +3030,7 @@ static int host_pfn_mapping_level(struct kvm *kvm, gfn_t gfn,
>
> int kvm_mmu_max_mapping_level(struct kvm *kvm,
> const struct kvm_memory_slot *slot, gfn_t gfn,
> - int max_level)
> + int max_level, bool is_private)
> {
> struct kvm_lpage_info *linfo;
> int host_level;
> @@ -3042,6 +3042,9 @@ 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;
>
> @@ -3070,7 +3073,8 @@ void kvm_mmu_hugepage_adjust(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault
> * 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->gfn, fault->max_level,
> + fault->is_private);
> if (fault->req_level == PG_LEVEL_4K || fault->huge_page_disallowed)
> return;
>
> @@ -4141,6 +4145,32 @@ 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);
> }
>
> +static inline u8 order_to_level(int order)
> +{
> + BUILD_BUG_ON(KVM_MAX_HUGEPAGE_LEVEL > PG_LEVEL_1G);
> +
> + 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 int kvm_faultin_pfn_private(struct kvm_page_fault *fault)
> +{
> + int order;
> + struct kvm_memory_slot *slot = fault->slot;
> +
> + if (kvm_restricted_mem_get_pfn(slot, fault->gfn, &fault->pfn, &order))
>+ return RET_PF_RETRY;
>+
>+ fault->max_level = min(order_to_level(order), fault->max_level);
>+ fault->map_writable = !(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;
>@@ -4173,6 +4203,22 @@ static int kvm_faultin_pfn(struct kvm_vcpu *vcpu, struct kvm_page_fault *fault)
> return RET_PF_EMULATE;
> }
>
>+ if (kvm_slot_can_be_private(slot) &&
>+ fault->is_private != kvm_mem_is_private(vcpu->kvm, fault->gfn)) {
>+ vcpu->run->exit_reason = KVM_EXIT_MEMORY_FAULT;
>+ if (fault->is_private)
>+ vcpu->run->memory.flags = KVM_MEMORY_EXIT_FLAG_PRIVATE;
>+ else
>+ vcpu->run->memory.flags = 0;
>+ vcpu->run->memory.padding = 0;
>+ vcpu->run->memory.gpa = fault->gfn << PAGE_SHIFT;
>+ vcpu->run->memory.size = PAGE_SIZE;
>+ return RET_PF_USER;
>+ }
>+
>+ if (fault->is_private)
>+ return kvm_faultin_pfn_private(fault);
>+

Since each memslot may also not be backed by restricted memory, we
should also check if the memslot has been set up for private memory
with

if (fault->is_private && kvm_slot_can_be_private(slot))
return kvm_faultin_pfn_private(fault);

Without this check, restrictedmem_get_page will get called with NULL
in slot->restricted_file, which causes a NULL pointer dereference.

> async = false;
> fault->pfn = __gfn_to_pfn_memslot(slot, fault->gfn, false, &async,
> fault->write, &fault->map_writable,
>@@ -5557,6 +5603,9 @@ int noinline kvm_mmu_page_fault(struct kvm_vcpu *vcpu, gpa_t cr2_or_gpa, u64 err
> return -EIO;
> }
>
>+ if (r == RET_PF_USER)
>+ return 0;
>+
> if (r < 0)
> return r;
> if (r != RET_PF_EMULATE)
>@@ -6408,7 +6457,8 @@ static bool kvm_mmu_zap_collapsible_spte(struct kvm *kvm,
> */
> if (sp->role.direct &&
> sp->role.level < kvm_mmu_max_mapping_level(kvm, slot, sp->gfn,
>- PG_LEVEL_NUM)) {
>+ PG_LEVEL_NUM,
>+ false)) {
> kvm_zap_one_rmap_spte(kvm, rmap_head, sptep);
>
> if (kvm_available_flush_tlb_with_range())