On Tue, Aug 09, 2016 at 08:44:53AM -0500, Bjorn Helgaas wrote:
> [+cc Lukas]
>
> On Tue, Aug 09, 2016 at 01:22:30PM +0200, Thomas Gleixner wrote:
> > From: Benedikt Spranger <b.spran...@linutronix.de>
> >
> > PCI and PCIBIOS probing only scans devices at function number 0/8/16/...
> > Subdevices (e.g. multiqueue) have function numbers which are not a
> > multiple of 8.
> >
> > Simple hypervisors (e.g. Jailhouse) pass subdevices directly w/o providing
> > virtual PCI mappings like KVM. As a consequence a simple PCI passthrough
> > from
> > Jailhouse to a linux guest is not able to detect such devices.
> >
> > Changing the probe functions to scan all function numbers makes it work.
> > This
> > has no side effects and there is no reason to force the 0/8/16... probing
> > scheme.
>
> "devfn" here is a 8-bit field (5 bits of device number and 3 bits of
> function number), so incrementing by 8 is really a way of looking at
> function 0 of each device number. I'm pretty sure this is based on
> something in the spec that says a multi-function device must implement
> function 0. Please look that up and include a reference in the
> changelog so we have a more complete story here.
>
> It's possible there are other assumptions in the code about
> multi-function devices always having a function 0. It would take a
> little more research to be certain that this wouldn't break anything.
>
> As Lukas pointed out, it does increase the number of probe attempts by
> a factor of 8. I don't know how much that will affect boot time, but
> it's certainly something to consider and hopefully quantify.
Any comments on this? I'm waiting for at least the spec reference
and hopefully some warm fuzzies about boot time and safety.
I looked up the spec: PCI (not PCIe) r3.0, sec 3.2.2.3.4, says:
A single-function device may optionally respond to all function
numbers as the same function or may ... respond only to function 0
and not respond to the other function numbers.
I'm concerned that a single-function device that responds to all
function numbers might break with this patch.
[multi-function devices] are also required to always implement
function 0 in the device.
Here's the reason we can advance by 8 in the "Go find them" loop.
If a single function device is detected (i.e., bit 7 in the Header
Type register of function 0 is 0), no more functions for that Device
Number will be checked. If a multi-function device is detected
(i.e., bit 7 in the Header Type register of function 0 is 1), then
all remaining Function Numbers will be checked.
This patch does the opposite of what the first sentence recommends.
> > Signed-off-by: Benedikt Spranger <b.spran...@linutronix.de>
> > Signed-off-by: Thomas Gleixner <t...@linutronix.de>
> > ---
> > arch/x86/pci/legacy.c | 2 +-
> > drivers/pci/probe.c | 2 +-
> > 2 files changed, 2 insertions(+), 2 deletions(-)
> >
> > --- a/arch/x86/pci/legacy.c
> > +++ b/arch/x86/pci/legacy.c
> > @@ -42,7 +42,7 @@ void pcibios_scan_specific_bus(int busn)
> > if (pci_find_bus(0, busn))
> > return;
> >
> > - for (devfn = 0; devfn < 256; devfn += 8) {
> > + for (devfn = 0; devfn < 256; devfn++) {
> > if (!raw_pci_read(0, busn, devfn, PCI_VENDOR_ID, 2, &l) &&
> > l != 0x0000 && l != 0xffff) {
> > DBG("Found device at %02x:%02x [%04x]\n", busn, devfn,
> > l);
> > --- a/drivers/pci/probe.c
> > +++ b/drivers/pci/probe.c
> > @@ -2063,7 +2063,7 @@ unsigned int pci_scan_child_bus(struct p
> > dev_dbg(&bus->dev, "scanning bus\n");
> >
> > /* Go find them, Rover! */
> > - for (devfn = 0; devfn < 0x100; devfn += 8)
> > + for (devfn = 0; devfn < 0x100; devfn++)
> > pci_scan_slot(bus, devfn);
> >
> > /* Reserve buses for SR-IOV capability. */
> > --
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