Re: [PATCH v3 1/3] PCI: Introduce pcibios_ignore_alignment_request

[Shawn Anastasio]

On 5/28/19 1:27 AM, Alexey Kardashevskiy wrote:
> On 28/05/2019 15:36, Oliver wrote:
> > On Tue, May 28, 2019 at 2:03 PM Shawn Anastasio <shawn@xxxxxxxxxx> wrote:
> > > Introduce a new pcibios function pcibios_ignore_alignment_request
> > > which allows the PCI core to defer to platform-specific code to
> > > determine whether or not to ignore alignment requests for PCI resources.
> > >
> > > The existing behavior is to simply ignore alignment requests when
> > > PCI_PROBE_ONLY is set. This is behavior is maintained by the
> > > default implementation of pcibios_ignore_alignment_request.
> > >
> > > Signed-off-by: Shawn Anastasio <shawn@xxxxxxxxxx>
> > > ---
> > >   drivers/pci/pci.c   | 9 +++++++--
> > >   include/linux/pci.h | 1 +
> > >   2 files changed, 8 insertions(+), 2 deletions(-)
> > >
> > > diff --git a/drivers/pci/pci.c b/drivers/pci/pci.c
> > > index 8abc843b1615..8207a09085d1 100644
> > > --- a/drivers/pci/pci.c
> > > +++ b/drivers/pci/pci.c
> > > @@ -5882,6 +5882,11 @@ resource_size_t __weak pcibios_default_alignment(void)
> > >          return 0;
> > >   }
> > >
> > > +int __weak pcibios_ignore_alignment_request(void)
> > > +{
> > > +       return pci_has_flag(PCI_PROBE_ONLY);
> > > +}
> > > +
> > >   #define RESOURCE_ALIGNMENT_PARAM_SIZE COMMAND_LINE_SIZE
> > >   static char resource_alignment_param[RESOURCE_ALIGNMENT_PARAM_SIZE] = {0};
> > >   static DEFINE_SPINLOCK(resource_alignment_lock);
> > > @@ -5906,9 +5911,9 @@ static resource_size_t pci_specified_resource_alignment(struct pci_dev *dev,
> > >          p = resource_alignment_param;
> > >          if (!*p && !align)
> > >                  goto out;
> > > -       if (pci_has_flag(PCI_PROBE_ONLY)) {
> > > +       if (pcibios_ignore_alignment_request()) {
> > >                  align = 0;
> > > -               pr_info_once("PCI: Ignoring requested alignments (PCI_PROBE_ONLY)\n");
> > > +               pr_info_once("PCI: Ignoring requested alignments\n");
> > >                  goto out;
> > >          }
> >
> > I think the logic here is questionable to begin with. If the user has
> > explicitly requested re-aligning a resource via the command line then
> > we should probably do it even if PCI_PROBE_ONLY is set. When it breaks
> > they get to keep the pieces.
> >
> > That said, the real issue here is that PCI_PROBE_ONLY probably
> > shouldn't be set under qemu/kvm. Under the other hypervisor (PowerVM)
> > hotplugged devices are configured by firmware before it's passed to
> > the guest and we need to keep the FW assignments otherwise things
> > break. QEMU however doesn't do any BAR assignments and relies on that
> > being handled by the guest. At boot time this is done by SLOF, but
> > Linux only keeps SLOF around until it's extracted the device-tree.
> > Once that's done SLOF gets blown away and the kernel needs to do it's
> > own BAR assignments. I'm guessing there's a hack in there to make it
> > work today, but it's a little surprising that it works at all...
>
>
> The hack is to run a modified qemu-aware "/usr/sbin/rtas_errd" in the
> guest which receives an event from qemu (RAS_EPOW from
> /proc/interrupts), fetches device tree chunks (and as I understand it -
> they come with BARs from phyp but without from qemu) and writes "1" to
> "/sys/bus/pci/rescan" which calls pci_assign_resource() eventually:

Interesting. Does this mean that the PHYP hotplug path doesn't
call pci_assign_resource? If so it means the patch may not
break that platform after all, though it still may not be
the correct way of doing things.

> [c000000006e6f960] [c0000000005f62d4] pci_assign_resource+0x44/0x360
>
> [c000000006e6fa10] [c0000000005f8b54]
> assign_requested_resources_sorted+0x84/0x110
> [c000000006e6fa60] [c0000000005f9540] __assign_resources_sorted+0xd0/0x750
> [c000000006e6fb40] [c0000000005fb2e0]
> __pci_bus_assign_resources+0x80/0x280
> [c000000006e6fc00] [c0000000005fb95c]
> pci_assign_unassigned_bus_resources+0xbc/0x100
> [c000000006e6fc60] [c0000000005e3d74] pci_rescan_bus+0x34/0x60
>
> [c000000006e6fc90] [c0000000005f1ef4] rescan_store+0x84/0xc0
>
> [c000000006e6fcd0] [c00000000068060c] bus_attr_store+0x3c/0x60
>
> [c000000006e6fcf0] [c00000000037853c] sysfs_kf_write+0x5c/0x80
>
>
>
>
>
> > IIRC Sam Bobroff was looking at hotplug under pseries recently so he
> > might have something to add. He's sick at the moment, but I'll ask him
> > to take a look at this once he's back among the living
> >
> > > diff --git a/include/linux/pci.h b/include/linux/pci.h
> > > index 4a5a84d7bdd4..47471dcdbaf9 100644
> > > --- a/include/linux/pci.h
> > > +++ b/include/linux/pci.h
> > > @@ -1990,6 +1990,7 @@ static inline void pcibios_penalize_isa_irq(int irq, int active) {}
> > >   int pcibios_alloc_irq(struct pci_dev *dev);
> > >   void pcibios_free_irq(struct pci_dev *dev);
> > >   resource_size_t pcibios_default_alignment(void);
> > > +int pcibios_ignore_alignment_request(void);
> > >
> > >   #ifdef CONFIG_HIBERNATE_CALLBACKS
> > >   extern struct dev_pm_ops pcibios_pm_ops;
> > > --
> > > 2.20.1