Messages in this thread |  | | Date | Mon, 19 Sep 2022 17:18:03 -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 17:02, Alper Gun wrote: > On Mon, Sep 19, 2022 at 2:38 PM Tom Lendacky <thomas.lendacky@amd.com> wrote: >> >> 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 > > But the host code updates the index. It doesn't leave the loop because > rc is 0. The guest will think that it is successful. > rc = __snp_handle_page_state_change(vcpu, op, gpa, level); > if (rc) > goto out; > > Also the page state change request with MSR is not retried. It > terminates the VM if the MSR request fails.
Ah, right. I see what you mean. It should probably return a -EAGAIN instead of 0 and then the if (rc) check should be modified to specifically look for -EAGAIN and goto out after setting rc to 0.
But that does leave the MSR protocol open to the problem that you mention, so, yes, retry logic in snp_handle_page_state_change() for a -EAGAIN seems reasonable.
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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