Re: [PATCH v2 07/28] KVM: arm64: Don't evaluate HCR_EL2.NV nor HFGITR_EL2.ERET on ERET fast path

[Marc Zyngier <maz@kernel.org>]

On Wed, 15 Jul 2026 09:00:37 +0100,
Oliver Upton <oupton@kernel.org> wrote:
> 
> Hey,
> 
> On Tue, Jul 14, 2026 at 10:16:20AM +0100, Marc Zyngier wrote:
> > We currently avoid using the ERET fast path if the guest has HCR_EL2.NV
> > set. This is an odd check, as NV doesn't mean much if HCR_EL2.TGE==1.
> > 
> > Similarly, evaluating HFGITR_EL2.ERET makes little sense, as
> > this only applies to the nested context, while the ERET fast-path
> > is purely for the benefit of L1.
> > 
> > Replace these bizarre checks with is_nested_ctxt() which makes a lot
> > more sense: if we are running an L2, the ERET trap must go to L1.
> > 
> > Fixes: dd0717a998f77 ("KVM: arm64: nv: Fast-track 'InHost' exception returns")
> > Signed-off-by: Marc Zyngier <maz@kernel.org>
> > ---
> >  arch/arm64/kvm/hyp/vhe/switch.c | 9 +++------
> >  1 file changed, 3 insertions(+), 6 deletions(-)
> > 
> > diff --git a/arch/arm64/kvm/hyp/vhe/switch.c b/arch/arm64/kvm/hyp/vhe/switch.c
> > index bbe9cebd3d9d5..a83be345f8450 100644
> > --- a/arch/arm64/kvm/hyp/vhe/switch.c
> > +++ b/arch/arm64/kvm/hyp/vhe/switch.c
> > @@ -344,13 +344,10 @@ static bool kvm_hyp_handle_eret(struct kvm_vcpu *vcpu, u64 *exit_code)
> >  	 * if this is a VHE guest hypervisor returning to its own
> >  	 * userspace, or the hypervisor performing a local exception
> >  	 * return. No need to save/restore registers, no need to
> > -	 * switch S2 MMU. Just do the canonical ERET.
> > -	 *
> > -	 * Unless the trap has to be forwarded further down the line,
> > -	 * of course...
> > +	 * switch S2 MMU. Just do the canonical ERET unlesss we are in
> > +	 * nested context.
> >  	 */
> 
> typo: unless
> 
> I feel like it wouldn't hurt to add a bit more detail to the comment
> here, that L0 KVM doesn't use ERET traps in the nested context.
> Otherwise it isn't clear why these always belong to the L1.

We already have a tiny bit of information in kvm_handle_eret(), but
that's obviously a bit far down the line. Something like this?

diff --git a/arch/arm64/kvm/hyp/vhe/switch.c b/arch/arm64/kvm/hyp/vhe/switch.c
index a83be345f8450..684cebf23aa0e 100644
--- a/arch/arm64/kvm/hyp/vhe/switch.c
+++ b/arch/arm64/kvm/hyp/vhe/switch.c
@@ -344,8 +344,13 @@ static bool kvm_hyp_handle_eret(struct kvm_vcpu *vcpu, u64 *exit_code)
 	 * if this is a VHE guest hypervisor returning to its own
 	 * userspace, or the hypervisor performing a local exception
 	 * return. No need to save/restore registers, no need to
-	 * switch S2 MMU. Just do the canonical ERET unlesss we are in
+	 * switch S2 MMU. Just do the canonical ERET unless we are in
 	 * nested context.
+	 *
+	 * Note that this is made possible because KVM itself never traps
+	 * ERET when running an L2. The consequence is that any ERET trap is
+	 * the result of HCR_EL2 or HFGITR_EL2 programming by L1 for its own
+	 * guest, and the exception must be forwarded to L1.
 	 */
 	if (is_nested_ctxt(vcpu))
 		return false;

Thanks,

	M.

-- 
Without deviation from the norm, progress is not possible.