Re: [PATCH Part2 v5 41/45] KVM: SVM: Add support to handle the RMP nested page fault

Dr. David Alan Gilbert
2021-09-29T12:25:05Z

* Brijesh Singh (brijesh.singh@amd.com) wrote:
> When SEV-SNP is enabled in the guest, the hardware places restrictions on
> all memory accesses based on the contents of the RMP table. When hardware
> encounters RMP check failure caused by the guest memory access it raises
> the #NPF. The error code contains additional information on the access
> type. See the APM volume 2 for additional information.
>
> Signed-off-by: Brijesh Singh <brijesh.singh@amd.com>
> ---
> arch/x86/kvm/svm/sev.c | 76 ++++++++++++++++++++++++++++++++++++++++++
> arch/x86/kvm/svm/svm.c | 14 +++++---
> arch/x86/kvm/svm/svm.h | 1 +
> 3 files changed, 87 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 65b578463271..712e8907bc39 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -3651,3 +3651,79 @@ void sev_post_unmap_gfn(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn, int token)
>
> srcu_read_unlock(&sev->psc_srcu, token);
> }
> +
> +void handle_rmp_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code)
> +{
> + int rmp_level, npt_level, rc, assigned;
> + struct kvm *kvm = vcpu->kvm;
> + gfn_t gfn = gpa_to_gfn(gpa);
> + bool need_psc = false;
> + enum psc_op psc_op;
> + kvm_pfn_t pfn;
> + bool private;
> +
> + write_lock(&kvm->mmu_lock);
> +
> + if (unlikely(!kvm_mmu_get_tdp_walk(vcpu, gpa, &pfn, &npt_level)))
> + goto unlock;
> +
> + assigned = snp_lookup_rmpentry(pfn, &rmp_level);
> + if (unlikely(assigned < 0))
> + goto unlock;
> +
> + private = !!(error_code & PFERR_GUEST_ENC_MASK);
> +
> + /*
> + * If the fault was due to size mismatch, or NPT and RMP page level's
> + * are not in sync, then use PSMASH to split the RMP entry into 4K.
> + */
> + if ((error_code & PFERR_GUEST_SIZEM_MASK) ||
> + (npt_level == PG_LEVEL_4K && rmp_level == PG_LEVEL_2M && private)) {
> + rc = snp_rmptable_psmash(kvm, pfn);
> + if (rc)
> + pr_err_ratelimited("psmash failed, gpa 0x%llx pfn 0x%llx rc %d\n",
> + gpa, pfn, rc);
> + goto out;
> + }
> +
> + /*
> + * If it's a private access, and the page is not assigned in the
> + * RMP table, create a new private RMP entry. This can happen if
> + * guest did not use the PSC VMGEXIT to transition the page state
> + * before the access.
> + */
> + if (!assigned && private) {
> + need_psc = 1;
> + psc_op = SNP_PAGE_STATE_PRIVATE;
> + goto out;
> + }
> +
> + /*
> + * If it's a shared access, but the page is private in the RMP table
> + * then make the page shared in the RMP table. This can happen if
> + * the guest did not use the PSC VMGEXIT to transition the page
> + * state before the access.
> + */
> + if (assigned && !private) {
> + need_psc = 1;
> + psc_op = SNP_PAGE_STATE_SHARED;
> + }
> +
> +out:
> + write_unlock(&kvm->mmu_lock);
> +
> + if (need_psc)
> + rc = __snp_handle_page_state_change(vcpu, psc_op, gpa, PG_LEVEL_4K);

That 'rc' never goes anywhere - should it?

> + /*
> + * The fault handler has updated the RMP pagesize, zap the existing
> + * rmaps for large entry ranges so that nested page table gets rebuilt
> + * with the updated RMP pagesize.
> + */
> + gfn = gpa_to_gfn(gpa) & ~(KVM_PAGES_PER_HPAGE(PG_LEVEL_2M) - 1);
> + kvm_zap_gfn_range(kvm, gfn, gfn + PTRS_PER_PMD);
> + return;
> +
> +unlock:
> + write_unlock(&kvm->mmu_lock);
> +}
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index 3784d389247b..3ba62f21b113 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1933,15 +1933,21 @@ static int pf_interception(struct kvm_vcpu *vcpu)
> static int npf_interception(struct kvm_vcpu *vcpu)
> {
> struct vcpu_svm *svm = to_svm(vcpu);
> + int rc;
>
> u64 fault_address = svm->vmcb->control.exit_info_2;
> u64 error_code = svm->vmcb->control.exit_info_1;
>
> trace_kvm_page_fault(fault_address, error_code);
> - return kvm_mmu_page_fault(vcpu, fault_address, error_code,
> - static_cpu_has(X86_FEATURE_DECODEASSISTS) ?
> - svm->vmcb->control.insn_bytes : NULL,
> - svm->vmcb->control.insn_len);
> + rc = kvm_mmu_page_fault(vcpu, fault_address, error_code,
> + static_cpu_has(X86_FEATURE_DECODEASSISTS) ?
> + svm->vmcb->control.insn_bytes : NULL,
> + svm->vmcb->control.insn_len);

If kvm_mmu_page_fault failed, (rc!=0) do you still want to call your
handler?

Dave

> + if (error_code & PFERR_GUEST_RMP_MASK)
> + handle_rmp_page_fault(vcpu, fault_address, error_code);
> +
> + return rc;
> }
>
> static int db_interception(struct kvm_vcpu *vcpu)
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index ff91184f9b4a..280072995306 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -626,6 +626,7 @@ 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, int *token);
> void sev_post_unmap_gfn(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn, int token);
> +void handle_rmp_page_fault(struct kvm_vcpu *vcpu, gpa_t gpa, u64 error_code);
>
> /* vmenter.S */
>
> --
> 2.17.1
>
>
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK