Messages in this thread |  | | Date | Mon, 19 Sep 2022 23:46:55 +0000 | Subject | Re: [PATCH Part2 v6 37/49] KVM: SVM: Add support to handle MSR based Page State Change VMGEXIT | From | Ashish Kalra <> |
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On 9/19/22 22:18, Tom Lendacky wrote: > 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
I believe it makes more sense to add the retry logic within __snp_handle_page_state_change() itself, as that will make it work for both the GHCB MSR protocol and the GHCB VMGEXIT requests.
Thanks, Ashish
> >> >>> >>>> 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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