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

Alper Gun
2022-09-19T17:54:11Z

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
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;
> > +}
> > +