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

Alper Gun
2022-09-19T22:03:08Z

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.

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