Re: [PATCH v2 1/2] PCI/MSI: Fix the confusing IRQ sysfs ABI for MSI-X

From: Barry Song
Date: Wed Aug 25 2021 - 05:30:17 EST

On Wed, Aug 25, 2021 at 8:51 AM Barry Song <21cnbao@xxxxxxxxx> wrote:
>
> On Sat, Aug 21, 2021 at 10:42 PM Marc Zyngier <maz@xxxxxxxxxx> wrote:
> >
> > Hi Bjorn,
> >
> > On Sat, 21 Aug 2021 00:33:28 +0100,
> > Bjorn Helgaas <helgaas@xxxxxxxxxx> wrote:
> > >
> > > [+cc Thomas, Marc]
> > >
> > > On Sat, Aug 21, 2021 at 10:37:43AM +1200, Barry Song wrote:
> > > > From: Barry Song <song.bao.hua@xxxxxxxxxxxxx>
> > > >
> > > > /sys/bus/pci/devices/.../irq sysfs ABI is very confusing at this
> > > > moment especially for MSI-X cases.
> > >
> > > AFAICT this patch *only* affects MSI-X. So are you saying the sysfs
> > > ABI is fine for MSI but confusing for MSI-X?
> > >
> > > > While MSI sets IRQ to the first
> > > > number in the vector, MSI-X does nothing for this though it saves
> > > > default_irq in msix_setup_entries(). Weird the saved default_irq
> > > > for MSI-X is never used in pci_msix_shutdown(), which is quite
> > > > different with pci_msi_shutdown(). Thus, this patch moves to show
> > > > the first IRQ number which is from the first msi_entry for MSI-X.
> > > > Hopefully, this can make IRQ ABI more clear and more consistent.
> > > >
> > > > Acked-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
> > > > Signed-off-by: Barry Song <song.bao.hua@xxxxxxxxxxxxx>
> > > > ---
> > > > drivers/pci/msi.c | 6 ++++++
> > > > 1 file changed, 6 insertions(+)
> > > >
> > > > diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
> > > > index 9232255..6bbf81b 100644
> > > > --- a/drivers/pci/msi.c
> > > > +++ b/drivers/pci/msi.c
> > > > @@ -771,6 +771,7 @@ static int msix_capability_init(struct pci_dev *dev, struct msix_entry *entries,
> > > > int ret;
> > > > u16 control;
> > > > void __iomem *base;
> > > > + struct msi_desc *desc;
> > > >
> > > > /* Ensure MSI-X is disabled while it is set up */
> > > > pci_msix_clear_and_set_ctrl(dev, PCI_MSIX_FLAGS_ENABLE, 0);
> > > > @@ -814,6 +815,10 @@ static int msix_capability_init(struct pci_dev *dev, struct msix_entry *entries,
> > > > pci_msix_clear_and_set_ctrl(dev, PCI_MSIX_FLAGS_MASKALL, 0);
> > > >
> > > > pcibios_free_irq(dev);
> > > > +
> > > > + desc = first_pci_msi_entry(dev);
> > > > + dev->irq = desc->irq;
> > >
> > > This change is not primarily about sysfs. This is about changing
> > > "dev->irq" when MSI-X is enabled, and it's only incidental that sysfs
> > > reflects that.
> > >
> > > So we need to know the effect of changing dev->irq. Drivers may use
> > > the value of dev->irq, and I'm *guessing* this change shouldn't break
> > > them since we already do this for MSI, but I'd like some more expert
> > > opinion than mine :)
> > >
> > > For MSI we have:
> > >
> > > msi_capability_init
> > > msi_setup_entry
> > > entry = alloc_msi_entry(nvec)
> > > entry->msi_attrib.default_irq = dev->irq; /* Save IOAPIC IRQ */
> > > dev->irq = entry->irq;
> > >
> > > pci_msi_shutdown
> > > /* Restore dev->irq to its default pin-assertion IRQ */
> > > dev->irq = desc->msi_attrib.default_irq;
> > >
> > > and for MSI-X we have:
> > >
> > > msix_capability_init
> > > msix_setup_entries
> > > for (i = 0; i < nvec; i++)
> > > entry = alloc_msi_entry(1)
> > > entry->msi_attrib.default_irq = dev->irq;
> > >
> > > pci_msix_shutdown
> > > for_each_pci_msi_entry(entry, dev)
> > > __pci_msix_desc_mask_irq
> > > + dev->irq = entry->msi_attrib.default_irq; # added by this patch
> > >
> > >
> > > Things that seem strange to me:
> > >
> > > - The msi_setup_entry() comment "Save IOAPIC IRQ" seems needlessly
> > > specific; maybe it should be "INTx IRQ".
> > >
> > > - The pci_msi_shutdown() comment "Restore ... pin-assertion IRQ"
> > > should match the msi_setup_entry() one, e.g., maybe it should also
> > > be "INTx IRQ". There are no INTx or IOAPIC pins in PCIe.
> > >
> > > - The only use of .default_irq is to save and restore dev->irq, so
> > > it looks like a per-device thing, not a per-vector thing.
> > >
> > > In msi_setup_entry() there's only one msi_entry, so there's only
> > > one saved .default_irq.
> > >
> > > In msix_setup_entries(), we get nvecs msi_entry structs, and we
> > > get a saved .default_irq in each one?
> >
> > That's a key point.
> >
> > Old-school PCI/MSI is represented by a single interrupt, and you
> > *could* somehow make it relatively easy for drivers that only
> > understand INTx to migrate to MSI if you replaced whatever is held in
> > dev->irq (which should only represent the INTx mapping) with the MSI
> > interrupt number. Which I guess is what the MSI code is doing.
> >
> > This is the 21st century, and nobody should ever rely on such horror,
> > but I'm sure we do have such drivers in the tree. Boo.
> >
> > However, this *cannot* hold true for Multi-MSI, nor MSI-X, because
> > there is a plurality of interrupts. Even worse, for MSI-X, there is
> > zero guarantee that the allocated interrupts will be in a contiguous
> > space.
> >
> > Given that, what is dev->irq good for? "Absolutely Nothing! (say it
> > again!)".
> >
> > MSI-X is not something you can "accidentally" use. You have to
> > actively embrace it. In all honesty, this patch tries to move in the
> > wrong direction. If anything, we should kill this hack altogether and
> > fix the (handful of?) drivers that rely on it. That'd actually be a
> > good way to find whether they are still worth keeping in the tree. And
> > if it breaks too many of them, then at least we'll know where we
> > stand.
> >
> > I'd be tempted to leave the below patch simmer in -next for a few
> > weeks and see if how many people shout:
> >
> > diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
> > index e5e75331b415..2be9a01cbe72 100644
> > --- a/drivers/pci/msi.c
> > +++ b/drivers/pci/msi.c
> > @@ -591,7 +591,6 @@ msi_setup_entry(struct pci_dev *dev, int nvec, struct irq_affinity *affd)
> > entry->msi_attrib.is_virtual = 0;
> > entry->msi_attrib.entry_nr = 0;
> > entry->msi_attrib.maskbit = !!(control & PCI_MSI_FLAGS_MASKBIT);
> > - entry->msi_attrib.default_irq = dev->irq; /* Save IOAPIC IRQ */
> > entry->msi_attrib.multi_cap = (control & PCI_MSI_FLAGS_QMASK) >> 1;
> > entry->msi_attrib.multiple = ilog2(__roundup_pow_of_two(nvec));
> >
> > @@ -682,7 +681,6 @@ static int msi_capability_init(struct pci_dev *dev, int nvec,
> > dev->msi_enabled = 1;
> >
> > pcibios_free_irq(dev);
> > - dev->irq = entry->irq;
> > return 0;
> > }
> >
> > @@ -742,7 +740,6 @@ static int msix_setup_entries(struct pci_dev *dev, void __iomem *base,
> > entry->msi_attrib.is_virtual =
> > entry->msi_attrib.entry_nr >= vec_count;
> >
> > - entry->msi_attrib.default_irq = dev->irq;
> > entry->mask_base = base;
> >
> > addr = pci_msix_desc_addr(entry);
> > @@ -964,8 +961,6 @@ static void pci_msi_shutdown(struct pci_dev *dev)
> > mask = msi_mask(desc->msi_attrib.multi_cap);
> > msi_mask_irq(desc, mask, 0);
> >
> > - /* Restore dev->irq to its default pin-assertion IRQ */
> > - dev->irq = desc->msi_attrib.default_irq;
> > pcibios_alloc_irq(dev);
> > }
> >
> > diff --git a/include/linux/msi.h b/include/linux/msi.h
> > index e8bdcb83172b..a631664c1c38 100644
> > --- a/include/linux/msi.h
> > +++ b/include/linux/msi.h
> > @@ -114,7 +114,6 @@ struct ti_sci_inta_msi_desc {
> > * @maskbit: [PCI MSI/X] Mask-Pending bit supported?
> > * @is_64: [PCI MSI/X] Address size: 0=32bit 1=64bit
> > * @entry_nr: [PCI MSI/X] Entry which is described by this descriptor
> > - * @default_irq:[PCI MSI/X] The default pre-assigned non-MSI irq
> > * @mask_pos: [PCI MSI] Mask register position
> > * @mask_base: [PCI MSI-X] Mask register base address
> > * @platform: [platform] Platform device specific msi descriptor data
> > @@ -148,7 +147,6 @@ struct msi_desc {
> > u8 is_64 : 1;
> > u8 is_virtual : 1;
> > u16 entry_nr;
> > - unsigned default_irq;
> > } msi_attrib;
> > union {
> > u8 mask_pos;
> >
>
> We will also need the below change as pci_irq_vector() depends on
> dev->irq for the MSI case.
>
> int pci_irq_vector(struct pci_dev *dev, unsigned int nr)
> {
> if (dev->msix_enabled) {
> struct msi_desc *entry;
> int i = 0;
>
> for_each_pci_msi_entry(entry, dev) {
> if (i == nr)
> return entry->irq;
> i++;
> }
> WARN_ON_ONCE(1);
> return -EINVAL;
> }
>
> if (dev->msi_enabled) {
> struct msi_desc *entry = first_pci_msi_entry(dev);
>
> if (WARN_ON_ONCE(nr >= entry->nvec_used))
> return -EINVAL;
>
> + return entry->irq + nr;
> } else {
> if (WARN_ON_ONCE(nr > 0))
> return -EINVAL;
> }
>
>
> - return dev->irq + nr;
> + return dev->irq;
> }
> EXPORT_SYMBOL(pci_irq_vector);
>

And here:

--- a/drivers/pci/msi.c
+++ b/drivers/pci/msi.c
@@ -401,7 +401,7 @@ static void __pci_restore_msi_state(struct pci_dev *dev)
if (!dev->msi_enabled)
return;

- entry = irq_get_msi_desc(dev->irq);
+ entry = first_pci_msi_entry(dev);

pci_intx_for_msi(dev, 0);
pci_msi_set_enable(dev, 0);

since drivers/net/ethernet/intel/ice/ice_virtchnl_pf.c is the only one
calling pci_restore_msi_state(),
will probably require one test from the driver maintainers.

> > Thanks,
> >
> > M.
> >
> > --
> > Without deviation from the norm, progress is not possible.
>
Thanks
barry