Re: [PATCH] iommu/amd: Fix a stale comment about which legacy mode is user visible

[Wei Wang]

On 4/7/26 7:03 AM, Sean Christopherson wrote:
> On Fri, Mar 14, 2025, Sean Christopherson wrote:
> > Update a stale comment about which of the legacy modes is visible to the
> > user, i.e. can be forced via amd_iommu_intr=legacy.
> >
> > Fixes: b74aa02d7a30 ("iommu/amd: Fix legacy interrupt remapping for x2APIC-enabled system")
> > Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
> > ---
> >   drivers/iommu/amd/amd_iommu_types.h | 11 ++++++-----
> >   1 file changed, 6 insertions(+), 5 deletions(-)
> >
> > diff --git a/drivers/iommu/amd/amd_iommu_types.h b/drivers/iommu/amd/amd_iommu_types.h
> > index 23caea22f8dc..f7877abd0091 100644
> > --- a/drivers/iommu/amd/amd_iommu_types.h
> > +++ b/drivers/iommu/amd/amd_iommu_types.h
> > @@ -968,12 +968,13 @@ static inline int get_hpet_devid(int id)
> >   }
> >   
> >   enum amd_iommu_intr_mode_type {
> > +	/*
> > +	 * The legacy format mode is not visible to users to prevent the user
> > +	 * from crashing x2APIC systems, which for all intents and purposes
> > +	 * require 128-bit IRTEs.   The legacy format will be forced as needed

Nit: Triple space before "The"

> > +	 * when hardware doesn't support 128-bit IRTEs.
> > +	 */
> >   	AMD_IOMMU_GUEST_IR_LEGACY,
> > -
> > -	/* This mode is not visible to users. It is used when
> > -	 * we cannot fully enable vAPIC and fallback to only support
> > -	 * legacy interrupt remapping via 128-bit IRTE.
> > -	 */
> >   	AMD_IOMMU_GUEST_IR_LEGACY_GA,
> >   	AMD_IOMMU_GUEST_IR_VAPIC,
> >   };
> >
> > base-commit: ea9bd29a9c0d757b3384ae3e633e6bbaddf00725
> > --
>
> High latency ping!  A year later, and this still applies cleanly :-)

The change looks good to me:
Reviewed-by: Wei Wang <wei.w.wang@xxxxxxxxxxx>


I think it's nicer to improve the kernel-parameters.txt as well:

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 03a550630644..68f7cdb606aa 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -459,7 +459,9 @@ Kernel parameters
        amd_iommu_intr= [HW,X86-64]
                        Specifies one of the following AMD IOMMU interrupt
                        remapping modes:
-                       legacy     - Use legacy interrupt remapping mode.
+                       legacy     - Use legacy (hypervisor-intercepted) interrupt
+                                    remapping mode. On modern systems, this safely
+                                    uses 128-bit IRTEs to support x2APIC.
                        vapic      - Use virtual APIC mode, which allows IOMMU
                                     to inject interrupts directly into guest.
                                     This mode requires kvm-amd.avic=1.