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    SubjectRe: [PATCH Part2 v6 37/49] KVM: SVM: Add support to handle MSR based Page State Change VMGEXIT
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    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;
    >>>>> +}
    >>>>> +

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