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Date: Mon, 15 Jan 2024 19:20:37 +0530
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Subject: Re: [PATCH V3] PCI/MSI: Fix MSI hwirq truncation
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To: Thomas Gleixner <tglx@linutronix.de>, bhelgaas@google.com,
 rdunlap@infradead.org, ilpo.jarvinen@linux.intel.com,
 jiang.liu@linux.intel.com
Cc: linux-pci@vger.kernel.org, linux-kernel@vger.kernel.org,
 treding@nvidia.com, jonathanh@nvidia.com, sdonthineni@nvidia.com,
 kthota@nvidia.com, mmaddireddy@nvidia.com, sagar.tv@gmail.com
References: <20240108120522.1368240-1-vidyas@nvidia.com>
 <20240111052814.713016-1-vidyas@nvidia.com> <87bk9qim4p.ffs@tglx>
 <b9a7b855-af2b-4858-b9d8-3340a0421cfe@nvidia.com> <877ckahq50.ffs@tglx>
From: Vidya Sagar <vidyas@nvidia.com>
In-Reply-To: <877ckahq50.ffs@tglx>
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On 1/15/2024 3:31 PM, Thomas Gleixner wrote:
> External email: Use caution opening links or attachments
> 
> 
> On Fri, Jan 12 2024 at 23:03, Vidya Sagar wrote:
>> On 1/12/2024 9:23 PM, Thomas Gleixner wrote:
>>> On Thu, Jan 11 2024 at 10:58, Vidya Sagar wrote:
>>>> So, cast the PCI domain number to 'irq_hw_number_t' before left shifting
>>>> it to calculate hwirq number.
>>>
>>> This still does not explain that this fixes it only on 64-bit platforms
>>> and why we don't care for 32-bit systems.
>> Agree that this fixes the issue only on 64-bit platforms. It doesn't
>> change the behavior on 32-bit platforms. My understanding is that the
>> issue surfaces only if there are too many PCIe controllers in the system
>> which usually is the case in modern server systems and it is arguable if
>> the server systems really run 32-bit kernels.
> 
> Arguably people who do that can keep the pieces.
> 
>> One way to fix it for both 32-bit and 64-bit systems is by changing the
>> type of 'hwirq' to u64. This may cause two memory reads in 32-bit
>> systems whenever 'hwirq' is accessed and that may intern cause some perf
>> impact?? Is this the way you think I should be handling it?
> 
> No. Leave it as is. What I'm asking for is that it's properly documented
> in the changelog.
Sure. I'll add this extra information in the change log.

> 
> Thanks,
> 
>          tglx
>