Re: [Xen-devel] [PATCH 6/6] xen-pt: Round pci regions sizes to XEN_PAGE_SIZE

[Simon Gaiser <simon@xxxxxxxxxxxxxxxxxxxxxx>]

Jason Andryuk:
> On Wed, Mar 13, 2019 at 11:09 AM Paul Durrant <Paul.Durrant@xxxxxxxxxx> wrote:
>>
>>> -----Original Message-----
>>> From: Jason Andryuk [mailto:jandryuk@xxxxxxxxx]
>>> Sent: 11 March 2019 18:02
>>> To: qemu-devel@xxxxxxxxxx
>>> Cc: xen-devel@xxxxxxxxxxxxxxxxxxxx; marmarek@xxxxxxxxxxxxxxxxxxxxxx; Simon 
>>> Gaiser
>>> <simon@xxxxxxxxxxxxxxxxxxxxxx>; Jason Andryuk <jandryuk@xxxxxxxxx>; Stefano 
>>> Stabellini
>>> <sstabellini@xxxxxxxxxx>; Anthony Perard <anthony.perard@xxxxxxxxxx>; Paul 
>>> Durrant
>>> <Paul.Durrant@xxxxxxxxxx>
>>> Subject: [PATCH 6/6] xen-pt: Round pci regions sizes to XEN_PAGE_SIZE
>>>
>>> From: Simon Gaiser <simon@xxxxxxxxxxxxxxxxxxxxxx>
>>>
>>> If a pci memory region has a size < XEN_PAGE_SIZE it can get located at
>>> an address which is not page aligned.
>>
>> IIRC the PCI spec says that the minimum memory region size should be at 
>> least 4k. Should we even be tolerating BARs smaller than that?
>>
>>   Paul
>>
> 
> Hi, Paul.
> 
> Simon found this, so it affects a real device.  Simon, do you recall
> which device was affected?
> 
> I think BARs only need to be power-of-two size and aligned, and 4k is
> not a minimum.  16bytes may be a minimum, but I don't know what the
> spec says.
> 
> On an Ivy Bridge system, here are some of the devices with BARs smaller than 
> 4K:
> 00:16.0 Communication controller: Intel Corporation 7 Series/C210
> Series Chipset Family MEI Controller #1 (rev 04)
>    Memory at d0735000 (64-bit, non-prefetchable) [disabled] [size=16]
> 00:1d.0 USB controller: Intel Corporation 7 Series/C210 Series Chipset
> Family USB Enhanced Host Controller #1 (rev 04) (prog-if 20 [EHCI])
>    Memory at d0739000 (32-bit, non-prefetchable) [disabled] [size=1K]
> 00:1f.3 SMBus: Intel Corporation 7 Series/C210 Series Chipset Family
> SMBus Controller (rev 04)
>    Memory at d0734000 (64-bit, non-prefetchable) [disabled] [size=256]
> 02:00.0 System peripheral: JMicron Technology Corp. SD/MMC Host
> Controller (rev 30)
>    Memory at d0503000 (32-bit, non-prefetchable) [disabled] [size=256]
> 
> These examples are all 4K aligned, so this is not an issue on this machine.

I wrote this patch quite some time ago, so I might be misremembering
something but IIRC the problem was the address qemu allocates with
memory_region_init_io(). I.e. the address as seen from inside the VM, so
it does not help if the real address is aligned.

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