x86/ioapic: Don't return 0 from arch_dynirq_lower_bound()

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, as 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")
Signed-off-by: Saurabh Sengar <ssengar@linux.microsoft.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/1679988604-20308-1-git-send-email-ssengar@linux.microsoft.com

diff --git a/arch/x86/kernel/apic/io_apic.c b/arch/x86/kernel/apic/io_apic.c
index 1f83b052bb74e0894abd128d4c71dcd4e5e80f96..f980b38b0227e29ba141935effbe631ebae0467b 100644 (file)
--- 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