Re: [PATCH v2] x86/ioapic: Don't return 0 from arch_dynirq_lower_bound()

[H. Peter Anvin]

On March 28, 2023 12:30:04 AM PDT, Saurabh Sengar <ssengar@xxxxxxxxxxxxxxxxxxx> 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@xxxxxxxxxxxxx>
>Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>Signed-off-by: Saurabh Sengar <ssengar@xxxxxxxxxxxxxxxxxxx>
>Cc: Andy Shevchenko <andriy.shevchenko@xxxxxxxxx>
>Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
>---
>[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?