linux-kernel - Re: [PATCH 0/3] PCI: Expose resource resizing through sysfs

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Date:   Thu, 18 Aug 2022 13:16:06 +0200
From:   Christian König <christian.koenig@....com>
To:     Alex Williamson <alex.williamson@...hat.com>
Cc:     linux-pci@...r.kernel.org, bhelgaas@...gle.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/3] PCI: Expose resource resizing through sysfs

Am 17.08.22 um 16:02 schrieb Alex Williamson:
> On Wed, 17 Aug 2022 12:10:44 +0200
> Christian König <christian.koenig@....com> wrote:
>
>> Am 16.08.22 um 21:39 schrieb Alex Williamson:
>>> We have a couple graphics drivers making use of PCIe Resizable BARs
>>> now, but I've been trying to figure out how we can make use of such
>>> features for devices assigned to a VM.  This is a proposal for a
>>> rather basic interface in sysfs such that we have the ability to
>>> pre-enable larger BARs before we bind devices to vfio-pci and
>>> attach them to a VM.
>> Ah, yes please.
>>
>> I was considering doing this myself just for testing while adding the
>> rebar support for the GFX drivers, but then just implementing it on the
>> GFX side was simpler.
>>
>> I would just add a warning that resizing BARs can easily crash the
>> system even when no driver directly claimed the resource or PCIe device.
>>
>> It literally took me weeks to figure out that I need to kick out the EFI
>> framebuffer driver before trying to resize the BAR or otherwise I just
>> get a hung system.
> Good point, I think maybe we can avoid crashing the system though if we
> use the new aperture support to remove conflicting drivers from all VGA
> class devices, similar to d17378062079 ("vfio/pci: Remove console
> drivers").  A note in the ABI documentation about removing console
> drivers from the device when resizing resources would definitely be in
> order.
>
>>> Along the way I found a double-free in the error path of creating
>>> resource attributes, that can certainly be pulled separately (1/).
>>>
>>> I'm using an RTX6000 for testing, which unexpectedly only supports
>>> REBAR with smaller than default sizes, which led me to question
>>> why we have such heavy requirements for shrinking resources (2/).
>> Oh, that's easy. You got tons of ARM boards with less than 512MiB of
>> address space per root PCIe complex.
>>
>> If you want to get a GPU working on those you need to decrease the
>> BAR size or otherwise you won't be able to fit 256MiB VRAM BAR +
>> register BAR into the same hole for the PCIe root complex.
>>
>> An alternative explanation is that at least AMD produced some boards
>> with a messed up resize configuration word. But on those you only got
>> 256MiB, 512MiB and 1GiB potential BAR sizes IIRC.
> An aspect of shrinking BARs that maybe I'm not giving enough
> consideration to is that we might be shrinking a BAR on one device in
> order to release MMIO space from a bridge aperture, that we might then
> use to expand a BAR elsewhere.  The RTX6000 case only frees a rather
> modest amount of MMIO space, but I could imagine more substantial
> configurations.  Maybe this justifies resizing the bridge aperture even
> in the shrinking case?

That was the original idea why I've kept that in there, yes.

But I never really seen a case where that really mattered.

So far making BARs smaller was something only the BIOS does.

Christian.

>
>> Anyway, with an appropriate warning added to the sysfs documentation
>> the patch #2 and #3 are Acked-by: Christian König
>> <christian.koenig@....com>
> Thanks!
>
> Alex
>