Re: [RFC PATCH kernel] Revert "net/mlx4_core: Set UAR page size to 4KB regardless of system page size"

Eli Cohen
2016-03-16T06:26:58Z

On Wed, Mar 16, 2016 at 04:49:00PM +1100, Alexey Kardashevskiy wrote:
> On 03/16/2016 04:10 PM, Eli Cohen wrote:
> >On Wed, Mar 16, 2016 at 01:07:58PM +1100, Alexey Kardashevskiy wrote:
> >>
> >>So with v4.5 as a host, there is no actual distro available today to
> >>use as a guest in the next 6 months (or whatever it takes to
> >>backport this partucular patch back there).
> >>
> >>You could have added a module parameter to enforce the old behavoir,
> >>at least...
> >>
> >>And sorry but from the original commit log I could not understand
> >>why exactly all existing guests need to be broken. Could you please
> >>point me to a piece of documentation describing all this UAR
> >>bisuness (what is UAR, why 128 UARs are required and for what, etc).
> >>Thanks.
> >>
> >
> >We are going to send a patch that fixes this using a module parameter.
> >The patch will be on top of Huy's patch.
> >
> >Some background to the problem: mlx4 supported devices require 128 UAR
>
> What does UAR stand for?
User Access Region. It's the way you interface with the hardware.
>
> >pages from PCI memory space defined by BAR2-3. Each UAR page can be
> >any power of 2 value from 4K up to 64K. Before Huy's patch the driver
> >chose UAR page size to be equal to system page size. Since PowerPC's
> >page size is 64K this means minimum requirement of UAR pages is not
> >met (default UAR BAR is 8MB and only half of it is really reserved for
> >UARs).
>
> And what was the downside? afaict the performance was good...
>

It's not a performance issue. Defining 64KB for a UAR is not required
and wastes pci memory mapped i/o space.

>
> >More details can be found in the programmer's manual.
>
> Can you please point me to this manual on the website? I tried,
> honestly, could not find it. Thanks.
>
It's not publically available. If you have an FAE that work with your
company you can ask him how to get the doc.
>
> --
> Alexey
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