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

From: Shawn Anastasio
Date: Sun Jun 02 2019 - 22:27:41 EST

On 5/30/19 10:56 PM, Alexey Kardashevskiy wrote:

On 31/05/2019 08:49, Shawn Anastasio wrote:
On 5/29/19 10:39 PM, Alexey Kardashevskiy wrote:

On 28/05/2019 17:39, Shawn Anastasio wrote:

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>

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

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>
ÂÂ drivers/pci/pci.cÂÂ | 9 +++++++--
ÂÂ include/linux/pci.h | 1 +
ÂÂ 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
ÂÂÂÂÂÂÂÂÂ return 0;
ÂÂ }

+int __weak pcibios_ignore_alignment_request(void)
+ÂÂÂÂÂÂ return pci_has_flag(PCI_PROBE_ONLY);
ÂÂ static char
resource_alignment_param[RESOURCE_ALIGNMENT_PARAM_SIZE] = {0};
ÂÂ static DEFINE_SPINLOCK(resource_alignment_lock);
@@ -5906,9 +5911,9 @@ static resource_size_t
pci_specified_resource_alignment(struct pci_dev *dev,
ÂÂÂÂÂÂÂÂÂ p = resource_alignment_param;
ÂÂÂÂÂÂÂÂÂ if (!*p && !align)
-ÂÂÂÂÂÂ if (pci_has_flag(PCI_PROBE_ONLY)) {
+ÂÂÂÂÂÂ if (pcibios_ignore_alignment_request()) {
-ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ pr_info_once("PCI: Ignoring requested alignments
+ÂÂÂÂÂÂÂÂÂÂÂÂÂÂ pr_info_once("PCI: Ignoring requested alignments\n");

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?

I'd expect dlpar_add_slot() to be called under phyp and eventually
pci_device_add() which (I think) may or may not trigger later

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.

We should probably stop enforcing the PCI_PROBE_ONLY flag - it seems
that (unless resource_alignment= is used) the pseries guest should just
walk through all allocated resources and leave them unchanged.

If we add a pcibios_default_alignment() implementation like was
suggested earlier, then it will behave as if the user has
specified resource_alignment= by default and SLOF's assignments
won't be honored (I think).

I removed pci_add_flags(PCI_PROBE_ONLY) from pSeries_setup_arch and
tried booting with and without pci=resource_alignment= and I can see no
difference - BARs are still aligned to 64K as programmed in SLOF; if I
hack SLOF to align to 4K or 32K - BARs get packed and the guest leaves
them unchanged.

I guess it boils down to one question - is it important that we
observe SLOF's initial BAR assignments?

It isn't if it's SLOF but it is if it's phyp. It used to not
allow/support BAR reassignment and even if it does not, I'd rather avoid
touching them.

A quick update. I tried removing pci_add_flags(PCI_PROBE_ONLY) which
worked, but if I add an implementation of pcibios_default_alignment
which simply returns PAGE_SIZE, my VM fails to boot and many errors
from the virtio disk driver are printed to the console.

After some investigation, it seems that with pcibios_default_alignment
present, Linux will reallocate all resources provided by SLOF on
boot. I'm still not sure why exactly this causes the virtio driver
to fail, but it does indicate that there is a reason to keep
SLOF's initial assignments.

Anybody have an idea what's causing this?