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

Tom Lendacky
2022-09-19T22:18:18Z

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