Re: [PATCH v2 11/13] KVM: nSVM: Simplify nested_svm_vmrun()

Yosry Ahmed
2025-12-11T19:25:34Z

On Tue, Dec 09, 2025 at 08:11:41AM -0800, Sean Christopherson wrote:
> On Mon, Nov 10, 2025, Yosry Ahmed wrote:
> > Call nested_svm_merge_msrpm() from enter_svm_guest_mode() if called from
> > the VMRUN path, instead of making the call in nested_svm_vmrun(). This
> > simplifies the flow of nested_svm_vmrun() and removes all jumps to
> > cleanup labels.
> >
> > Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
> > ---
> > arch/x86/kvm/svm/nested.c | 28 +++++++++++++---------------
> > 1 file changed, 13 insertions(+), 15 deletions(-)
> >
> > diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> > index a48668c36a191..89830380cebc5 100644
> > --- a/arch/x86/kvm/svm/nested.c
> > +++ b/arch/x86/kvm/svm/nested.c
> > @@ -1020,6 +1020,9 @@ int enter_svm_guest_mode(struct kvm_vcpu *vcpu, u64 vmcb12_gpa, bool from_vmrun)
> >
> > nested_svm_hv_update_vm_vp_ids(vcpu);
> >
> > + if (from_vmrun && !nested_svm_merge_msrpm(vcpu))
>
> This is silly, just do:
>
> if (enter_svm_guest_mode(vcpu, vmcb12_gpa, true) ||
> nested_svm_merge_msrpm(vcpu)) {
> svm->nested.nested_run_pending = 0;
> svm->nmi_l1_to_l2 = false;
> svm->soft_int_injected = false;
>
> svm->vmcb->control.exit_code = SVM_EXIT_ERR;
> svm->vmcb->control.exit_code_hi = -1u;
> svm->vmcb->control.exit_info_1 = 0;
> svm->vmcb->control.exit_info_2 = 0;
>
> nested_svm_vmexit(svm);
> }

Actually, if we go with the approach of making all VMRUN failures
happen before preparing the VMCB02 (as discussed in the other thread),
then we will want to call nested_svm_merge_msrpm() from within
enter_svm_guest_mode().

Otherwise, we either have a separate failure path for
nested_svm_merge_msrpm(), or we make all VMRUN failures happen after
preparing the VMCB02 and handled by nested_svm_vmexit().

I like having a separate exit path for VMRUN failures, and it makes more
sense to do the consistency checks on VMCB12 before preparing VMCB02.
But I understand if you prefer to keep things simple and move all
failures after VMCB02.

I already have it implemented with the separate VMRUN failure path, but
I don't wanna spam you with another series if you prefer it the other
way.

>
> > + return -1;
>
> Please stop returning -1, use a proper -errno.
>
> > +
> > return 0;
> > }
> >
> > @@ -1105,23 +1108,18 @@ int nested_svm_vmrun(struct kvm_vcpu *vcpu)
> >
> > svm->nested.nested_run_pending = 1;
> >
> > - if (enter_svm_guest_mode(vcpu, vmcb12_gpa, true))
> > - goto out_exit_err;
> > -
> > - if (nested_svm_merge_msrpm(vcpu))
> > - return ret;
> > -
> > -out_exit_err:
> > - svm->nested.nested_run_pending = 0;
> > - svm->nmi_l1_to_l2 = false;
> > - svm->soft_int_injected = false;
> > + if (enter_svm_guest_mode(vcpu, vmcb12_gpa, true)) {
> > + svm->nested.nested_run_pending = 0;
> > + svm->nmi_l1_to_l2 = false;
> > + svm->soft_int_injected = false;
> >
> > - svm->vmcb->control.exit_code = SVM_EXIT_ERR;
> > - svm->vmcb->control.exit_code_hi = 0;
> > - svm->vmcb->control.exit_info_1 = 0;
> > - svm->vmcb->control.exit_info_2 = 0;
> > + svm->vmcb->control.exit_code = SVM_EXIT_ERR;
> > + svm->vmcb->control.exit_code_hi = 0;
> > + svm->vmcb->control.exit_info_1 = 0;
> > + svm->vmcb->control.exit_info_2 = 0;
> >
> > - nested_svm_vmexit(svm);
> > + nested_svm_vmexit(svm);
>
> Note, there's a pre-existing bug in nested_svm_vmexit(). Lovely, and it's a
> user-triggerable WARN_ON() (and not even a WARN_ON_ONCE() at that).
>
> If nested_svm_vmexit() fails to map vmcb12, it (unbelievably stupidly) injects a
> #GP and hopes for the best. Oh FFS, it also has the asinine -EINVAL "logic".
> Anyways, it injects #GP (maybe), and bails early, which leaves
> KVM_REQ_GET_NESTED_STATE_PAGES set. KVM will then process that on the next
> vcpu_enter_guest() and trip the WARN_ON() in svm_get_nested_state_pages().
>
> Something like this to clean up the mess:
>
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index d4c872843a9d..96f8009a0d45 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -1018,9 +1018,6 @@ int enter_svm_guest_mode(struct kvm_vcpu *vcpu, u64 vmcb12_gpa, bool from_vmrun)
>
> nested_svm_hv_update_vm_vp_ids(vcpu);
>
> - if (from_vmrun && !nested_svm_merge_msrpm(vcpu))
> - return -1;
> -
> return 0;
> }
>
> @@ -1094,7 +1091,8 @@ int nested_svm_vmrun(struct kvm_vcpu *vcpu)
>
> svm->nested.nested_run_pending = 1;
>
> - if (enter_svm_guest_mode(vcpu, vmcb12_gpa, true)) {
> + if (enter_svm_guest_mode(vcpu, vmcb12_gpa, true) ||
> + nested_svm_merge_msrpm(vcpu)) {
> svm->nested.nested_run_pending = 0;
> svm->nmi_l1_to_l2 = false;
> svm->soft_int_injected = false;
> @@ -1158,24 +1156,16 @@ void svm_copy_vmloadsave_state(struct vmcb *to_vmcb, struct vmcb *from_vmcb)
> int nested_svm_vmexit(struct vcpu_svm *svm)
> {
> struct kvm_vcpu *vcpu = &svm->vcpu;
> + gpa_t vmcb12_gpa = svm->nested.vmcb12_gpa;
> struct vmcb *vmcb01 = svm->vmcb01.ptr;
> struct vmcb *vmcb02 = svm->nested.vmcb02.ptr;
> struct vmcb *vmcb12;
> struct kvm_host_map map;
> - int rc;
> -
> - rc = kvm_vcpu_map(vcpu, gpa_to_gfn(svm->nested.vmcb12_gpa), &map);
> - if (rc) {
> - if (rc == -EINVAL)
> - kvm_inject_gp(vcpu, 0);
> - return 1;
> - }
>
> vmcb12 = map.hva;
>
> /* Exit Guest-Mode */
> leave_guest_mode(vcpu);
> - svm->nested.vmcb12_gpa = 0;
> WARN_ON_ONCE(svm->nested.nested_run_pending);
>
> kvm_clear_request(KVM_REQ_GET_NESTED_STATE_PAGES, vcpu);
> @@ -1183,6 +1173,13 @@ int nested_svm_vmexit(struct vcpu_svm *svm)
> /* in case we halted in L2 */
> kvm_set_mp_state(vcpu, KVM_MP_STATE_RUNNABLE);
>
> + svm->nested.vmcb12_gpa = 0;
> +
> + if (kvm_vcpu_map(vcpu, gpa_to_gfn(vmcb12_gpa), &map)) {
> + kvm_make_request(KVM_REQ_TRIPLE_FAULT, vcpu);
> + return 1;
> + }
> +
> /* Give the current vmcb to the guest */
>
> vmcb12->save.es = vmcb02->save.es;
> @@ -1973,7 +1970,7 @@ static int svm_set_nested_state(struct kvm_vcpu *vcpu,
>
> static bool svm_get_nested_state_pages(struct kvm_vcpu *vcpu)
> {
> - if (WARN_ON(!is_guest_mode(vcpu)))
> + if (WARN_ON_ONCE(!is_guest_mode(vcpu)))
> return true;
>
> if (!vcpu->arch.pdptrs_from_userspace &&
>