On Wed, Mar 05, 2025 at 11:30:51AM +0100, Jan Beulich wrote:
> On 28.02.2025 12:32, Roger Pau Monne wrote:
> > @@ -191,8 +193,6 @@ void msi_compose_msg(unsigned vector, const cpumask_t
> > *cpu_mask, struct msi_msg
> >
> > static int write_msi_msg(struct msi_desc *entry, struct msi_msg *msg)
> > {
> > - entry->msg = *msg;
> > -
> > if ( iommu_intremap != iommu_intremap_off )
> > {
> > int rc;
> > @@ -203,6 +203,20 @@ static int write_msi_msg(struct msi_desc *entry,
> > struct msi_msg *msg)
> > return rc;
> > }
> >
> > + /*
> > + * Avoid updating the MSI entry if the address and data fields haven't
> > + * changed. When using interrupt remapping changing the MSI affinity
> > + * shouldn't change the interrupt remapping table index, and hence the
> > MSI
> > + * address and data fields should remain the same.
> > + */
> > + if ( entry->msg.address == msg->address && entry->msg.data ==
> > msg->data )
> > + {
> > + entry->msg.dest32 = msg->dest32;
> > + return 0;
> > + }
> > +
> > + entry->msg = *msg;
>
> It is perhaps pure luck that iommu_update_ire_from_msi() doesn't use entry's
> "msg" field, and hence that this re-arrangement is okay. It's unclear to me
> whether going forward this might not bite us.
I've updated the comment in msi_desc to make it clear what the `msg`
field contains. If iommu_update_ire_from_msi() has a need to fetch
the previous non-translated data and address fields it could always
add an extra field to msi_desc.
> > @@ -1407,7 +1415,9 @@ int pci_restore_msi_state(struct pci_dev *pdev)
> > }
> > type = entry->msi_attrib.type;
> >
> > - msg = entry->msg;
> > + msg.dest32 = entry->msg.dest32;
> > + msi_compose_msg(desc->arch.vector, NULL, &msg);
> > + entry->msg = (typeof(entry->msg)){};
> > write_msi_msg(entry, &msg);
>
> Hmm, this isn't exactly a "restore" then anymore. That said, re-constructing
> the message may even be more correct. Then, however, the question is whether
> passing NULL as msi_compose_msg()'s middle argument is really appropriate. A
> little bit of commentary may be desirable here in any event, also as to need
> to clear entry->msg.
I can add a comment. Note that as part of the patch a comment is
already added to both the msi_compose_msg() prototype and definition
regarding what passing a NULL cpu_mask implies.
>
> There's (at least) one place where behavior changes with the change of what
> we store in struct msi_desc's msg field (previously untranslated, now
> translated): dump_msi() wants to use the untranslated form. I fear it can't
> even re-construct some of the data it means to log (without reading from
> the IRTE).
Oh, I've missed dump_msi(). Let me see what I can do there.
Another possibility is for iommu_update_ire_from_msi() to report
whether the IRTE index has changed, and so the MSI fields have been
updated and need propagating to the hardware.
> > --- a/xen/drivers/passthrough/vtd/iommu.c
> > +++ b/xen/drivers/passthrough/vtd/iommu.c
> > @@ -1182,7 +1182,7 @@ static void cf_check dma_msi_end(struct irq_desc
> > *desc, u8 vector)
> > static void cf_check dma_msi_set_affinity(
> > struct irq_desc *desc, const cpumask_t *mask)
> > {
> > - struct msi_msg msg;
> > + struct msi_msg msg = {};
> > unsigned int dest;
> > unsigned long flags;
> > struct vtd_iommu *iommu = desc->action->dev_id;
>
> Why not a similar transformation as you do in set_msi_affinity(), eliminating
> the local "dest"?
It was more intrusive, but I can certainly do it.
> A change like the one here is likely needed in __hpet_setup_msi_irq(), to
> prevent accidental "uninitialized struct field" warnings.
Hm, won't the struct be fully initialized in that case, by getting
passed a cpu_mask? I don't mind doing so however.
> hpet_msi_set_affinity() might then also want to use msi_compose_msg(), albeit
> that may also be regarded as an independent change.
Possibly, note that HPET code doesn't use write_msi_msg(), and hence
is only partially affected by the msi_compose_msg() change, but not
the write_msi_msg() one, as it continues to store the untranslated MSI
address and data fields in hpet_event_channel struct.
Thanks, Roger.
