linux-kernel - Re: [PATCH v2] x86/ioapic: Don't return 0 from arch_dynirq_lower

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Message-ID: <20230411060946.GA17134@linuxonhyperv3.guj3yctzbm1etfxqx2vob5hsef.xx.internal.cloudapp.net>
Date:   Mon, 10 Apr 2023 23:09:46 -0700
From:   Saurabh Singh Sengar <ssengar@...ux.microsoft.com>
To:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, x86@...nel.org, hpa@...or.com,
        johan+linaro@...nel.org, isaku.yamahata@...el.com,
        mikelley@...rosoft.com, linux-kernel@...r.kernel.org,
        andriy.shevchenko@...el.com
Subject: Re: [PATCH v2] x86/ioapic: Don't return 0 from
 arch_dynirq_lower_bound()

On Tue, Mar 28, 2023 at 12:30:04AM -0700, Saurabh Sengar 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@...utronix.de>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Saurabh Sengar <ssengar@...ux.microsoft.com>
> Cc: Andy Shevchenko <andriy.shevchenko@...el.com>
> Cc: Thomas Gleixner <tglx@...utronix.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 this good to get accepted ? Please let me know if anything pending from my end on this.

- Saurabh

> -- 
> 2.34.1