Re: [PATCH] vfio/pci: Virtualize Maximum Payload Size
From: Auger Eric
Date: Wed Sep 20 2017 - 10:26:34 EST
On 20/09/2017 15:01, Sinan Kaya wrote:
> On 9/20/2017 3:59 AM, Auger Eric wrote:
>>> My impression is that MRRS is predominantly device and driver
>>> dependent, not topology dependent. A device can send a read request
>>> with a size larger than MPS, which implies that the device supplying
>>> the read data would split it into multiple TLPs based on MPS.
>> I read that too on the net. However in in 188.8.131.52. (3.0. Nov 10), Rules
>> for SW Configuration it is written:
>> "Software must set Max_Read_Request_Size of an isochronous-configured
>> device with a value that does not exceed the Max_Payload_Size set for
>> the device."
>> But on the the other hand some drivers are setting the MMRS directly
>> without further checking the MPS?
> We discussed this on LPC. MRRS and MPS are two independent concepts and
> are not related to each other under normal circumstances.
> The only valid criteria is that MRRS needs to be a multiple of MPS.
> Because completions are required to be a minimum of MPS size. If MRRS > MPS,
> read response is sent as multiple completions.
With that patch, you can end up with MRRS < MPS. Do I understand
correctly this is an issue?
> The only reason you want to match MRRS==MPS is that you don't want a single
> device to hog system resources. You can have MRRS 4k and MPS 128 bytes. Completions
> come in as 128 x N packets on the PCI bus.
> If you are sharing the same PCI bus with some other device, switch; you are effectively
> stalling other devices. That's why, isochronous devices are requesting small MRRS.
> An Isochronous device is an exception not norm.