linux-kernel - Re: [PATCH V3] PCI/MSI: Fix MSI hwirq truncation

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Date: Mon, 15 Jan 2024 11:01:15 +0100
From: Thomas Gleixner <tglx@...utronix.de>
To: Vidya Sagar <vidyas@...dia.com>, bhelgaas@...gle.com,
 rdunlap@...radead.org, ilpo.jarvinen@...ux.intel.com,
 jiang.liu@...ux.intel.com
Cc: linux-pci@...r.kernel.org, linux-kernel@...r.kernel.org,
 treding@...dia.com, jonathanh@...dia.com, sdonthineni@...dia.com,
 kthota@...dia.com, mmaddireddy@...dia.com, sagar.tv@...il.com
Subject: Re: [PATCH V3] PCI/MSI: Fix MSI hwirq truncation

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.

Thanks,

        tglx