Re: [PATCH] iommu/amd: Fix event counter availability check

[Shuah Khan]

On 2/26/21 2:44 PM, Paul Menzel wrote:
> [cc: +suravee, +jörg]
>
> Dear Alex, dear Shuah, dear Suravee, dear Jörg,
>
>
> Am 03.06.20 um 08:54 schrieb Alexander Monakov:
> > On Tue, 2 Jun 2020, Shuah Khan wrote:
> >
> > > I changed the logic to read config to get max banks and counters
> > > before checking if counters are writable and tried writing to all.
> > > The result is the same and all of them aren't writable. However,
> > > when disable the writable check and assume they are, I can run
> > [snip]
> >
> > This is similar to what I did. I also noticed that counters can
> > be successfully used with perf if the initial check is ignored.
> > I was considering sending a patch to remove the check and adjust
> > the event counting logic to use counters as read-only, but after
> > a bit more investigation I've noticed how late pci_enable_device
> > is done, and came up with this patch. It's a path of less resistance:
> > I'd expect maintainers to be more averse to removing the check
> > rather than fixing it so it works as intended (even though I think
> > the check should not be there in the first place).
> >
> > However:
> >
> > The ability to modify the counters is needed only for sampling the
> > events (getting an interrupt when a counter overflows). There's no
> > code to do that for these AMD IOMMU counters. A solution I would
> > prefer is to not write to those counters at all. It would simplify or
> > even remove a bunch of code. I can submit a corresponding patch if
> > there's general agreement this path is ok.
> >
> > What do you think?
>
> I like this idea. Suravee, Jörg, what do you think?
>
> Commit 6778ff5b21b (iommu/amd: Fix performance counter initialization) 
> delays the boot up to 100 ms, which is over 20 % on fast systems, and 
> also just a workaround, and does not seem to work always. The delay is 
> also not mentioned in the commit message.

Sounds good to me. I can test this out on my system.

thanks,
-- Shuah