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

Tom Lendacky
2022-09-19T21:38:12Z

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

> 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;
>>> +}
>>> +