Messages in this thread |  | | Date | Wed, 12 Oct 2022 15:15:15 -0500 | Subject | Re: [PATCH Part2 v6 37/49] KVM: SVM: Add support to handle MSR based Page State Change VMGEXIT | From | "Kalra, Ashish" <> |
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On 9/26/2022 10:19 AM, Peter Gonda wrote: > On Mon, Sep 19, 2022 at 5:47 PM Ashish Kalra <ashkalra@amd.com> wrote: >> >> >> 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. > > You are suggesting we just retry 'kvm_mmu_get_tdp_walk' inside of > __snp_handle_page_state_change()? That should work but how many times > do we retry? If we return EAGAIN or error we can leave it up to the > caller >
Ok, we return -EAGAIN here and then let the caller in snp_handle_page_state_change() or sev_handle_vmgexit_msr_protocol() (in case of GHCB MSR protocol) do the retries.
But, the question still remains, how may retry attempts should the caller attempt ?
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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