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

From: Tom Lendacky
Date: Mon Sep 19 2022 - 18:18:18 EST

On 9/19/22 17:02, Alper Gun wrote:
On Mon, Sep 19, 2022 at 2:38 PM Tom Lendacky <thomas.lendacky@xxxxxxx> wrote:

On 9/19/22 12:53, Alper Gun wrote:
On Fri, Aug 19, 2022 at 9:54 AM Peter Gonda <pgonda@xxxxxxxxxx> 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;
+}
+