linux-kernel - Re: [PATCH v2] x86/ioapic: Don't return 0 from arch_dynirq_lower

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Date:   Wed, 12 Apr 2023 17:37:27 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Saurabh Singh Sengar <ssengar@...ux.microsoft.com>,
        "H. Peter Anvin" <hpa@...or.com>
Cc:     mingo@...hat.com, bp@...en8.de, dave.hansen@...ux.intel.com,
        x86@...nel.org, johan+linaro@...nel.org, isaku.yamahata@...el.com,
        mikelley@...rosoft.com, linux-kernel@...r.kernel.org,
        andriy.shevchenko@...el.com
Subject: Re: [PATCH v2] x86/ioapic: Don't return 0 from
 arch_dynirq_lower_bound()

On Tue, Mar 28 2023 at 07:48, Saurabh Singh Sengar wrote:
> On Tue, Mar 28, 2023 at 06:59:04AM -0700, H. Peter Anvin wrote:
>> 
>> Is there any reason why this variable can't be initialized to a fixed nonzero number, like 16?
>
> Yes, initializing gst_top to any non-zero value should fix this issue.
> At first I thought to intialize gst_top to 1.

That works only in your case. Some boot time registrations of IO_APICs
use gsi_top as the base. So initializing gsi_top to N would move IOAPIC[0]
interrupts out to irq N... and make the legacy interrupts fail.

That whole IOAPIC registration could do with some major cleanup, but
that's a different story.

Thanks,

        tglx