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

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Date:   Mon, 27 May 2019 23:09:43 -0500
From:   Shawn Anastasio <shawn@...stas.io>
To:     Oliver <oohall@...il.com>
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 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.

>> 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
>>