Re: [PATCH Part2 v6 39/49] KVM: SVM: Introduce ops for the post gfn map and unmap

[Alper Gun]

On Mon, Jun 20, 2022 at 4:12 PM Ashish Kalra <Ashish.Kalra@xxxxxxx> wrote:
>
> From: Brijesh Singh <brijesh.singh@xxxxxxx>
>
> When SEV-SNP is enabled in the guest VM, the guest memory pages can
> either be a private or shared. A write from the hypervisor goes through
> the RMP checks. If hardware sees that hypervisor is attempting to write
> to a guest private page, then it triggers an RMP violation #PF.
>
> To avoid the RMP violation with GHCB pages, added new post_{map,unmap}_gfn
> functions to verify if its safe to map GHCB pages.  Uses a spinlock to
> protect against the page state change for existing mapped pages.
>
> Need to add generic post_{map,unmap}_gfn() ops that can be used to verify
> that its safe to map a given guest page in the hypervisor.
>
> This patch will need to be revisited later after consensus is reached on
> how to manage guest private memory as probably UPM private memslots will
> be able to handle this page state change more gracefully.
>
> Signed-off-by: Brijesh Singh <brijesh.singh@xxxxxxx>
> Signed-off by: Ashish Kalra <ashish.kalra@xxxxxxx>
> ---
>  arch/x86/include/asm/kvm-x86-ops.h |  1 +
>  arch/x86/include/asm/kvm_host.h    |  3 ++
>  arch/x86/kvm/svm/sev.c             | 48 ++++++++++++++++++++++++++++--
>  arch/x86/kvm/svm/svm.c             |  3 ++
>  arch/x86/kvm/svm/svm.h             | 11 +++++++
>  5 files changed, 64 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
> index e0068e702692..2dd2bc0cf4c3 100644
> --- a/arch/x86/include/asm/kvm-x86-ops.h
> +++ b/arch/x86/include/asm/kvm-x86-ops.h
> @@ -130,6 +130,7 @@ KVM_X86_OP(vcpu_deliver_sipi_vector)
>  KVM_X86_OP_OPTIONAL_RET0(vcpu_get_apicv_inhibit_reasons);
>  KVM_X86_OP(alloc_apic_backing_page)
>  KVM_X86_OP_OPTIONAL(rmp_page_level_adjust)
> +KVM_X86_OP(update_protected_guest_state)
>
>  #undef KVM_X86_OP
>  #undef KVM_X86_OP_OPTIONAL
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 49b217dc8d7e..8abc0e724f5c 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1522,7 +1522,10 @@ struct kvm_x86_ops {
>         unsigned long (*vcpu_get_apicv_inhibit_reasons)(struct kvm_vcpu *vcpu);
>
>         void *(*alloc_apic_backing_page)(struct kvm_vcpu *vcpu);
> +
>         void (*rmp_page_level_adjust)(struct kvm *kvm, kvm_pfn_t pfn, int *level);
> +
> +       int (*update_protected_guest_state)(struct kvm_vcpu *vcpu);
>  };
>
>  struct kvm_x86_nested_ops {
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index cb2d1bbb862b..4ed90331bca0 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -341,6 +341,7 @@ static int sev_guest_init(struct kvm *kvm, struct kvm_sev_cmd *argp)
>                 if (ret)
>                         goto e_free;
>
> +               spin_lock_init(&sev->psc_lock);
>                 ret = sev_snp_init(&argp->error);
>         } else {
>                 ret = sev_platform_init(&argp->error);
> @@ -2828,19 +2829,28 @@ static inline int svm_map_ghcb(struct vcpu_svm *svm, struct kvm_host_map *map)
>  {
>         struct vmcb_control_area *control = &svm->vmcb->control;
>         u64 gfn = gpa_to_gfn(control->ghcb_gpa);
> +       struct kvm_vcpu *vcpu = &svm->vcpu;
>
> -       if (kvm_vcpu_map(&svm->vcpu, gfn, map)) {
> +       if (kvm_vcpu_map(vcpu, gfn, map)) {
>                 /* Unable to map GHCB from guest */
>                 pr_err("error mapping GHCB GFN [%#llx] from guest\n", gfn);
>                 return -EFAULT;
>         }
>
> +       if (sev_post_map_gfn(vcpu->kvm, map->gfn, map->pfn)) {
> +               kvm_vcpu_unmap(vcpu, map, false);
> +               return -EBUSY;
> +       }
> +
>         return 0;
>  }
>
>  static inline void svm_unmap_ghcb(struct vcpu_svm *svm, struct kvm_host_map *map)
>  {
> -       kvm_vcpu_unmap(&svm->vcpu, map, true);
> +       struct kvm_vcpu *vcpu = &svm->vcpu;
> +
> +       kvm_vcpu_unmap(vcpu, map, true);
> +       sev_post_unmap_gfn(vcpu->kvm, map->gfn, map->pfn);
>  }
>
>  static void dump_ghcb(struct vcpu_svm *svm)
> @@ -3383,6 +3393,8 @@ static int __snp_handle_page_state_change(struct kvm_vcpu *vcpu, enum psc_op op,
>                                 return PSC_UNDEF_ERR;
>                 }
>
> +               spin_lock(&sev->psc_lock);
> +
>                 write_lock(&kvm->mmu_lock);
>
>                 rc = kvm_mmu_get_tdp_walk(vcpu, gpa, &pfn, &npt_level);
> @@ -3417,6 +3429,8 @@ static int __snp_handle_page_state_change(struct kvm_vcpu *vcpu, enum psc_op op,
>
>                 write_unlock(&kvm->mmu_lock);
>
> +               spin_unlock(&sev->psc_lock);

There is a corner case where the psc_lock is not released. If
kvm_mmu_get_tdp_walk fails, the lock will be kept and will cause soft
lockup.

> +
>                 if (rc) {
>                         pr_err_ratelimited("Error op %d gpa %llx pfn %llx level %d rc %d\n",
>                                            op, gpa, pfn, level, rc);
> @@ -3965,3 +3979,33 @@ void sev_rmp_page_level_adjust(struct kvm *kvm, kvm_pfn_t pfn, int *level)
>         /* Adjust the level to keep the NPT and RMP in sync */
>         *level = min_t(size_t, *level, rmp_level);
>  }
> +
> +int sev_post_map_gfn(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn)
> +{
> +       struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> +       int level;
> +
> +       if (!sev_snp_guest(kvm))
> +               return 0;
> +
> +       spin_lock(&sev->psc_lock);
> +
> +       /* If pfn is not added as private then fail */
> +       if (snp_lookup_rmpentry(pfn, &level) == 1) {
> +               spin_unlock(&sev->psc_lock);
> +               pr_err_ratelimited("failed to map private gfn 0x%llx pfn 0x%llx\n", gfn, pfn);
> +               return -EBUSY;
> +       }
> +
> +       return 0;
> +}
> +
> +void sev_post_unmap_gfn(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn)
> +{
> +       struct kvm_sev_info *sev = &to_kvm_svm(kvm)->sev_info;
> +
> +       if (!sev_snp_guest(kvm))
> +               return;
> +
> +       spin_unlock(&sev->psc_lock);
> +}
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index b24e0171cbf2..1c8e035ba011 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -4734,7 +4734,10 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
>         .vcpu_get_apicv_inhibit_reasons = avic_vcpu_get_apicv_inhibit_reasons,
>
>         .alloc_apic_backing_page = svm_alloc_apic_backing_page,
> +
>         .rmp_page_level_adjust = sev_rmp_page_level_adjust,
> +
> +       .update_protected_guest_state = sev_snp_update_protected_guest_state,
>  };
>
>  /*
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 54ff56cb6125..3fd95193ed8d 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -79,19 +79,25 @@ struct kvm_sev_info {
>         bool active;            /* SEV enabled guest */
>         bool es_active;         /* SEV-ES enabled guest */
>         bool snp_active;        /* SEV-SNP enabled guest */
> +
>         unsigned int asid;      /* ASID used for this guest */
>         unsigned int handle;    /* SEV firmware handle */
>         int fd;                 /* SEV device fd */
> +
>         unsigned long pages_locked; /* Number of pages locked */
>         struct list_head regions_list;  /* List of registered regions */
> +
>         u64 ap_jump_table;      /* SEV-ES AP Jump Table address */
> +
>         struct kvm *enc_context_owner; /* Owner of copied encryption context */
>         struct list_head mirror_vms; /* List of VMs mirroring */
>         struct list_head mirror_entry; /* Use as a list entry of mirrors */
>         struct misc_cg *misc_cg; /* For misc cgroup accounting */
>         atomic_t migration_in_progress;
> +
>         u64 snp_init_flags;
>         void *snp_context;      /* SNP guest context page */
> +       spinlock_t psc_lock;
>  };
>
>  struct kvm_svm {
> @@ -702,6 +708,11 @@ void sev_es_prepare_switch_to_guest(struct sev_es_save_area *hostsa);
>  void sev_es_unmap_ghcb(struct vcpu_svm *svm);
>  struct page *snp_safe_alloc_page(struct kvm_vcpu *vcpu);
>  void sev_rmp_page_level_adjust(struct kvm *kvm, kvm_pfn_t pfn, int *level);
> +int sev_post_map_gfn(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn);
> +void sev_post_unmap_gfn(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn);
> +void handle_rmp_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code);
> +void sev_snp_init_protected_guest_state(struct kvm_vcpu *vcpu);
> +int sev_snp_update_protected_guest_state(struct kvm_vcpu *vcpu);
>
>  /* vmenter.S */
>
> --
> 2.25.1
>