linux-kernel - Re: [RESEND PATCH 0/3] Allow custom PCI resource alignment on pseries

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Message-ID: <CAOSf1CG9LcxPy9cOk0FeNq=pvMCPKVKYU7=tdrMSO+RiqTWr5A@mail.gmail.com>
Date:   Tue, 28 May 2019 14:52:35 +1000
From:   Oliver <oohall@...il.com>
To:     Shawn Anastasio <shawn@...stas.io>
Cc:     linux-pci@...r.kernel.org,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Sam Bobroff <sbobroff@...ux.ibm.com>,
        xyjxie@...ux.vnet.ibm.com, rppt@...ux.ibm.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Alexey Kardashevskiy <aik@...abs.ru>
Subject: Re: [RESEND PATCH 0/3] Allow custom PCI resource alignment on pseries

On Tue, May 28, 2019 at 2:09 PM Shawn Anastasio <shawn@...stas.io> wrote:
>
>
>
> On 5/27/19 11:01 PM, Oliver wrote:
> > On Tue, May 28, 2019 at 8:56 AM Shawn Anastasio <shawn@...stas.io> wrote:
> >>
> >> Hello all,
> >>
> >> This patch set implements support for user-specified PCI resource
> >> alignment on the pseries platform for hotplugged PCI devices.
> >> Currently on pseries, PCI resource alignments specified with the
> >> pci=resource_alignment commandline argument are ignored, since
> >> the firmware is in charge of managing the PCI resources. In the
> >> case of hotplugged devices, though, the kernel is in charge of
> >> configuring the resources and should obey alignment requirements.
> >
> > Are you using hotplug to work around SLOF (the OF we use under qemu)
> > not aligning BARs to 64K? It looks like there is a commit in SLOF to
> > fix that (https://git.qemu.org/?p=SLOF.git;a=commit;f=board-qemu/slof/pci-phb.fs;h=1903174472f8800caf50c959b304501b4c01153c).
> >
>
> No, my application actually requires PCI hotplug at run-time.
>
> >> The current behavior of ignoring the alignment for hotplugged devices
> >> results in sub-page BARs landing between page boundaries and
> >> becoming un-mappable from userspace via the VFIO framework.
> >> This issue was observed on a pseries KVM guest with hotplugged
> >> ivshmem devices.
> >
> >> With these changes, users can specify an appropriate
> >> pci=resource_alignment argument on boot for devices they wish to use
> >> with VFIO.
> >>
> >> In the future, this could be extended to provide page-aligned
> >> resources by default for hotplugged devices, similar to what is done
> >> on powernv by commit 382746376993 ("powerpc/powernv: Override
> >> pcibios_default_alignment() to force PCI devices to be page aligned").
> >
> > Can we make aligning the BARs to PAGE_SIZE the default behaviour? The
> > BAR assignment process is complex enough as-is so I'd rather we didn't
> > add another platform hack into the mix.
>
> Absolutely. This will still require the existing changes so that the
> custom alignment isn't flat-out ignored on pseries, but I can set
> it to default to PAGE_SIZE as well, similar to how it's done on PowerNV.
> I've just pushed a v3 to fix a typo and I'll incorporate this change
> in v4.

I was thinking we could get rid of the ppcmd callback and do it in
kernel/pci-common.c. PowerNV is the only platform that implements the
callback and the pseries implementation is going to be identical so I
don't think there's much of point in keeping the callback.

> >> Feedback is appreciated.
> >>
> >> Thanks,
> >> Shawn
> >>
> >> Shawn Anastasio (3):
> >>    PCI: Introduce pcibios_ignore_alignment_request
> >>    powerpc/64: Enable pcibios_after_init hook on ppc64
> >>    powerpc/pseries: Allow user-specified PCI resource alignment after
> >>      init
> >>
> >>   arch/powerpc/include/asm/machdep.h     |  6 ++++--
> >>   arch/powerpc/kernel/pci-common.c       |  9 +++++++++
> >>   arch/powerpc/kernel/pci_64.c           |  4 ++++
> >>   arch/powerpc/platforms/pseries/setup.c | 22 ++++++++++++++++++++++
> >>   drivers/pci/pci.c                      |  9 +++++++--
> >>   5 files changed, 46 insertions(+), 4 deletions(-)
> >>
> >> --
> >> 2.20.1
> >>