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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Saurabh Singh Sengar <ssengar@linux.microsoft.com>,
        "H. Peter Anvin" <hpa@zytor.com>
Cc:     mingo@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com,
        x86@kernel.org, johan+linaro@kernel.org, isaku.yamahata@intel.com,
        mikelley@microsoft.com, linux-kernel@vger.kernel.org,
        andriy.shevchenko@intel.com
Subject: Re: [PATCH v2] x86/ioapic: Don't return 0 from
 arch_dynirq_lower_bound()
In-Reply-To: <20230328144834.GA17868@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
References: <1679988604-20308-1-git-send-email-ssengar@linux.microsoft.com>
 <3D028B51-35D7-435F-B912-DC5C0D6C36C6@zytor.com>
 <20230328144834.GA17868@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
Date:   Wed, 12 Apr 2023 17:37:27 +0200
Message-ID: <87v8i1yuvc.ffs@tglx>
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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