Re: [PATCH Part2 v6 37/49] KVM: SVM: Add support to handle MSR based Page State Change VMGEXIT

Ashish Kalra
2022-09-19T23:47:08Z

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;
>>>>>> +}
>>>>>> +