On Sun, Aug 22, 2021 at 10:41:17AM +1200, Barry Song wrote: > On Sun, Aug 22, 2021 at 10:14 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!)". > > > > > > > The only thing is that dev->irq is an sysfs ABI to userspace. Due to > > the inconsistency > > between legacy PCI INTx, MSI, MSI-X, this ABI should have been > > absolutely broken nowadays. > > This is actually what the patchset was originally aiming at to fix. > > > > One more question from me is that does dev->irq actually hold any > > valid hardware INTx > > information while hardware is using MSI-X? At least in my hardware, > > sysfs ABI for PCI is all "0". > > > > root@ubuntu:/sys/devices/pci0000:7c/0000:7c:00.0/0000:7d:00.3# cat irq > > 0 > > > > root@ubuntu:/sys/devices/pci0000:7c/0000:7c:00.0/0000:7d:00.3# ls -l msi_irqs/* > > -r--r--r-- 1 root root 4096 Aug 21 22:04 msi_irqs/499 > > -r--r--r-- 1 root root 4096 Aug 21 22:04 msi_irqs/500 > > -r--r--r-- 1 root root 4096 Aug 21 22:04 msi_irqs/501 > > ... > > root@ubuntu:/sys/devices/pci0000:7c/0000:7c:00.0/0000:7d:00.3# cat msi_irqs/499 > > msix > > > > Not quite sure how it is going on different hardware platforms. > > > > > 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: > > > > This looks like a more proper direction to go. > > but here i am wondering how sysfs ABI document should follow the below change > > doc is patch 2/2: > > https://lore.kernel.org/lkml/20210820223744.8439-3-21cnbao@xxxxxxxxx/ > > > > On the other hand, my feeling is that nobody should depend on sysfs > > irq entry nowadays. > > For example, userspace irqbalance is actually using /sys/devices/.../msi_irqs/ > > So probably we should set this ABI invisible when devices are using > > MSI or MSI-X? > > i mean something like the below, > > diff --git a/drivers/pci/pci-sysfs.c b/drivers/pci/pci-sysfs.c > index 5d63df7..1323841 100644 > --- a/drivers/pci/pci-sysfs.c > +++ b/drivers/pci/pci-sysfs.c > @@ -26,6 +26,7 @@ > #include <linux/slab.h> > #include <linux/vgaarb.h> > #include <linux/pm_runtime.h> > +#include <linux/msi.h> > #include <linux/of.h> > #include "pci.h" > > @@ -1437,6 +1438,16 @@ static umode_t pci_dev_attrs_are_visible(struct > kobject *kobj, > if ((pdev->class >> 8) != PCI_CLASS_DISPLAY_VGA) > return 0; > > +#ifdef CONFIG_PCI_MSI > + /* > + * if devices are MSI and MSI-X, IRQ sysfs ABI is meaningless > + * and broken > + */ > + if (a == &dev_attr_irq.attr) > + if (first_pci_msi_entry(pdev)) > + return 0; > +#endif > + > return a->mode; > } I think this idea has been discarded anyway, but sysfs doesn't work this way. The .is_visible() function is evaluated once at device_add()-time, i.e., during enumeration, so there's no way to dynamically change the visibility as the driver enables/disables MSI. I *wish* sysfs had more flexibility like this, though.
