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

From: Paul Menzel <pmenzel@molgen.mpg.de>
To: Alexander Monakov <amonakov@ispras.ru>,
	Shuah Khan <skhan@linuxfoundation.org>,
	Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Cc: iommu@lists.linux-foundation.org, linux-kernel@vger.kernel.org,
	David Coe <david.coe@live.co.uk>, Joerg Roedel <joro@8bytes.org>,
	"Tj (Elloe Linux)" <ml.linux@elloe.vision>
Subject: Re: [PATCH] iommu/amd: Fix event counter availability check
Date: Fri, 26 Feb 2021 22:44:08 +0100	[thread overview]
Message-ID: <4aba4c61-1878-3d4e-d52e-3ccac9715010@molgen.mpg.de> (raw)
In-Reply-To: <alpine.LNX.2.20.13.2006030935570.3181@monopod.intra.ispras.ru>

[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.

Kind regards,

Paul

[1]: 
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6778ff5b21bd8e78c8bd547fd66437cf2657fd9b