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

[Marc Zyngier]

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;

Thanks,

	M.

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