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Date:   Tue, 28 Mar 2023 07:48:34 -0700
From:   Saurabh Singh Sengar <ssengar@linux.microsoft.com>
To:     "H. Peter Anvin" <hpa@zytor.com>
Cc:     tglx@linutronix.de, 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()
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On Tue, Mar 28, 2023 at 06:59:04AM -0700, H. Peter Anvin wrote:
> On March 28, 2023 12:30:04 AM PDT, Saurabh Sengar <ssengar@linux.microsoft.com> wrote:
> >arch_dynirq_lower_bound() is invoked by the core interrupt code to
> >retrieve the lowest possible Linux interrupt number for dynamically
> >allocated interrupts like MSI.
> >
> >The x86 implementation uses this to exclude the IO/APIC GSI space.
> >This works correctly as long as there is an IO/APIC registered, but
> >returns 0 if not. This has been observed in VMs where the BIOS does
> >not advertise an IO/APIC.
> >
> >0 is an invalid interrupt number except for the legacy timer interrupt
> >on x86. The return value is unchecked in the core code, so it ends up
> >to allocate interrupt number 0 which is subsequently considered to be
> >invalid by the caller, e.g. the MSI allocation code.
> >
> >The function has already a check for 0 in the case that an IO/APIC is
> >registered, but ioapic_dynirq_base is 0 in case of device tree setups.
> >
> >Consolidate this and zero check for both ioapic_dynirq_base and gsi_top,
> >which is used in the case that no IO/APIC is registered.
> >
> >Fixes: 3e5bedc2c258 ("x86/apic: Fix arch_dynirq_lower_bound() bug for DT enabled machines")
> >Co-developed-by: Thomas Gleixner <tglx@linutronix.de>
> >Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> >Signed-off-by: Saurabh Sengar <ssengar@linux.microsoft.com>
> >Cc: Andy Shevchenko <andriy.shevchenko@intel.com>
> >Cc: Thomas Gleixner <tglx@linutronix.de>
> >---
> >[V2]
> >- Edit commit message
> >- Consolidated the 0 check for ioapic_dynirq_base as well
> >
> > arch/x86/kernel/apic/io_apic.c | 14 +++++++++-----
> > 1 file changed, 9 insertions(+), 5 deletions(-)
> >
> >diff --git a/arch/x86/kernel/apic/io_apic.c b/arch/x86/kernel/apic/io_apic.c
> >index 1f83b052bb74..f980b38b0227 100644
> >--- a/arch/x86/kernel/apic/io_apic.c
> >+++ b/arch/x86/kernel/apic/io_apic.c
> >@@ -2477,17 +2477,21 @@ static int io_apic_get_redir_entries(int ioapic)
> > 
> > unsigned int arch_dynirq_lower_bound(unsigned int from)
> > {
> >+	unsigned int ret;
> >+
> > 	/*
> > 	 * dmar_alloc_hwirq() may be called before setup_IO_APIC(), so use
> > 	 * gsi_top if ioapic_dynirq_base hasn't been initialized yet.
> > 	 */
> >-	if (!ioapic_initialized)
> >-		return gsi_top;
> >+	ret = ioapic_dynirq_base ? : gsi_top;
> >+
> > 	/*
> >-	 * For DT enabled machines ioapic_dynirq_base is irrelevant and not
> >-	 * updated. So simply return @from if ioapic_dynirq_base == 0.
> >+	 * For DT enabled machines ioapic_dynirq_base is irrelevant and
> >+	 * always 0. gsi_top can be 0 if there is no IO/APIC registered.
> >+	 * 0 is an invalid interrupt number for dynamic allocations. Return
> >+	 * @from instead.
> > 	 */
> >-	return ioapic_dynirq_base ? : from;
> >+	return ret ? : from;
> > }
> > 
> > #ifdef CONFIG_X86_32
> 
> 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. But then I looked at how
the ioapic_dynirq_base case is handled and followed a similar approach.

Regards,
Saurabh