linux-kernel - Re: [PATCH] iommu: amd: Fix IOMMU perf counter clobbering during init

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Date:   Thu, 23 Jan 2020 14:27:15 -0700
From:   Shuah Khan <skhan@...uxfoundation.org>
To:     Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
        Joerg Roedel <joro@...tes.org>
Cc:     iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org,
        Shuah Khan <skhan@...uxfoundation.org>
Subject: Re: [PATCH] iommu: amd: Fix IOMMU perf counter clobbering during init

On 1/21/20 8:32 AM, Shuah Khan wrote:
> On 1/20/20 7:10 PM, Suravee Suthikulpanit wrote:
>> On 1/17/2020 5:08 PM, Joerg Roedel wrote:
>>> Adding Suravee, who wrote the IOMMU Perf Counter code.
>>>
>>> On Tue, Jan 14, 2020 at 08:12:20AM -0700, Shuah Khan wrote:
>>>> init_iommu_perf_ctr() clobbers the register when it checks write access
>>>> to IOMMU perf counters and fails to restore when they are writable.
>>>>
>>>> Add save and restore to fix it.
>>>>
>>>> Signed-off-by: Shuah Khan<skhan@...uxfoundation.org>
>>>> ---
>>>>   drivers/iommu/amd_iommu_init.c | 22 ++++++++++++++++------
>>>>   1 file changed, 16 insertions(+), 6 deletions(-)
>>> Suravee, can you please review this patch?
>>>
>>
>> This looks ok. Does this fix certain issues? Or is this just for sanity.
> 
> I didn't notice any problems. Counters aren't writable on my system.
> However, it certainly looks like a bog since registers aren't restored
> like in other places in this file where such checks are done on other
> registers.
> 
> I see 2 banks and 4 counters on my system. Is it sufficient to check
> the first bank and first counter? In other words, if the first one
> isn't writable, are all counters non-writable?
> 
> Should we read the config first and then, try to see if any of the
> counters are writable? I have a patch that does that, I can send it
> out for review.
> 
>>
>> Reviewed-by: Suravee Suthikulpanit <suravee.suthikulpanit@....com>

Joerg,

Please don't pull this in. I introduced a bug in this patch. It always
returns amd_iommu_pc_present false even when it can write to the
counters. My bad.

I will send v2.

thanks,
-- Shuah