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

From: Shuah Khan <skhan@linuxfoundation.org>
To: Paul Menzel <pmenzel@molgen.mpg.de>,
	Alexander Monakov <amonakov@ispras.ru>,
	Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Cc: David Coe <david.coe@live.co.uk>,
	linux-kernel@vger.kernel.org, iommu@lists.linux-foundation.org,
	Shuah Khan <skhan@linuxfoundation.org>,
	"Tj \(Elloe Linux\)" <ml.linux@elloe.vision>
Subject: Re: [PATCH] iommu/amd: Fix event counter availability check
Date: Fri, 26 Feb 2021 14:55:16 -0700	[thread overview]
Message-ID: <e4e0861a-0a27-3765-5b63-161bb9bdb775@linuxfoundation.org> (raw)
In-Reply-To: <4aba4c61-1878-3d4e-d52e-3ccac9715010@molgen.mpg.de>

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

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