Patch "x86/ioapic: Don't return 0 from arch_dynirq_lower_bound()" has been added to the 4.14-stable tree

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This is a note to let you know that I've just added the patch titled

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

to the 4.14-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     x86-ioapic-don-t-return-0-from-arch_dynirq_lower_bou.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit 3d885d1dec96d5ca4722368c406d91865c8e817e
Author: Saurabh Sengar <ssengar@xxxxxxxxxxxxxxxxxxx>
Date:   Tue Mar 28 00:30:04 2023 -0700

    x86/ioapic: Don't return 0 from arch_dynirq_lower_bound()
    
    [ Upstream commit 5af507bef93c09a94fb8f058213b489178f4cbe5 ]
    
    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@xxxxxxxxxxxxxxxxxxx>
    Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
    Link: https://lore.kernel.org/r/1679988604-20308-1-git-send-email-ssengar@xxxxxxxxxxxxxxxxxxx
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/arch/x86/kernel/apic/io_apic.c b/arch/x86/kernel/apic/io_apic.c
index de74bca6a8ff6..1cceb30357aaf 100644
--- a/arch/x86/kernel/apic/io_apic.c
+++ b/arch/x86/kernel/apic/io_apic.c
@@ -2357,17 +2357,21 @@ 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



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