Messages in this thread | | | Date | Mon, 19 Sep 2022 16:38:02 -0500 | Subject | Re: [PATCH Part2 v6 37/49] KVM: SVM: Add support to handle MSR based Page State Change VMGEXIT | From | Tom Lendacky <> |
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On 9/19/22 12:53, Alper Gun wrote: > On Fri, Aug 19, 2022 at 9:54 AM Peter Gonda <pgonda@google.com> wrote: >> >>> + >>> +static int __snp_handle_page_state_change(struct kvm_vcpu *vcpu, enum psc_op op, gpa_t gpa, >>> + int level) >>> +{ >>> + struct kvm_sev_info *sev = &to_kvm_svm(vcpu->kvm)->sev_info; >>> + struct kvm *kvm = vcpu->kvm; >>> + int rc, npt_level; >>> + kvm_pfn_t pfn; >>> + gpa_t gpa_end; >>> + >>> + gpa_end = gpa + page_level_size(level); >>> + >>> + while (gpa < gpa_end) { >>> + /* >>> + * If the gpa is not present in the NPT then build the NPT. >>> + */ >>> + rc = snp_check_and_build_npt(vcpu, gpa, level); >>> + if (rc) >>> + return -EINVAL; >>> + >>> + if (op == SNP_PAGE_STATE_PRIVATE) { >>> + hva_t hva; >>> + >>> + if (snp_gpa_to_hva(kvm, gpa, &hva)) >>> + return -EINVAL; >>> + >>> + /* >>> + * Verify that the hva range is registered. This enforcement is >>> + * required to avoid the cases where a page is marked private >>> + * in the RMP table but never gets cleanup during the VM >>> + * termination path. >>> + */ >>> + mutex_lock(&kvm->lock); >>> + rc = is_hva_registered(kvm, hva, page_level_size(level)); >>> + mutex_unlock(&kvm->lock); >>> + if (!rc) >>> + return -EINVAL; >>> + >>> + /* >>> + * Mark the userspace range unmerable before adding the pages >>> + * in the RMP table. >>> + */ >>> + mmap_write_lock(kvm->mm); >>> + rc = snp_mark_unmergable(kvm, hva, page_level_size(level)); >>> + mmap_write_unlock(kvm->mm); >>> + if (rc) >>> + return -EINVAL; >>> + } >>> + >>> + write_lock(&kvm->mmu_lock); >>> + >>> + rc = kvm_mmu_get_tdp_walk(vcpu, gpa, &pfn, &npt_level); >>> + if (!rc) { >>> + /* >>> + * This may happen if another vCPU unmapped the page >>> + * before we acquire the lock. Retry the PSC. >>> + */ >>> + write_unlock(&kvm->mmu_lock); >>> + return 0; >>> + } >> >> I think we want to return -EAGAIN or similar if we want the caller to >> retry, right? I think returning 0 here hides the error. >> > > The problem here is that the caller(linux guest kernel) doesn't retry > if PSC fails. The current implementation in the guest kernel is that > if a page state change request fails, it terminates the VM with > GHCB_TERM_PSC reason. > Returning 0 here is not a good option because it will fail the PSC > silently and will probably cause a nested RMP fault later. Returning
Returning 0 here is ok because the PSC current index into the PSC structure will not be updated and the guest will then retry (see the loop in vmgexit_psc() in arch/x86/kernel/sev.c).
Thanks, Tom
> an error also terminates the guest immediately with current guest > implementation. I think the best approach here is adding a retry logic > to this function. Retrying without returning an error should help it > work because snp_check_and_build_npt will be called again and in the > second attempt this should work. > >>> + >>> + /* >>> + * Adjust the level so that we don't go higher than the backing >>> + * page level. >>> + */ >>> + level = min_t(size_t, level, npt_level); >>> + >>> + trace_kvm_snp_psc(vcpu->vcpu_id, pfn, gpa, op, level); >>> + >>> + switch (op) { >>> + case SNP_PAGE_STATE_SHARED: >>> + rc = snp_make_page_shared(kvm, gpa, pfn, level); >>> + break; >>> + case SNP_PAGE_STATE_PRIVATE: >>> + rc = rmp_make_private(pfn, gpa, level, sev->asid, false); >>> + break; >>> + default: >>> + rc = -EINVAL; >>> + break; >>> + } >>> + >>> + write_unlock(&kvm->mmu_lock); >>> + >>> + if (rc) { >>> + pr_err_ratelimited("Error op %d gpa %llx pfn %llx level %d rc %d\n", >>> + op, gpa, pfn, level, rc); >>> + return rc; >>> + } >>> + >>> + gpa = gpa + page_level_size(level); >>> + } >>> + >>> + return 0; >>> +} >>> +
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