Re: [PATCH 1/2] ns16550: reject IRQ above nr_irqs

[Jan Beulich <jbeulich@xxxxxxxx>]

On 10.03.2022 17:21, Julien Grall wrote:
> On 10/03/2022 16:12, Roger Pau Monné wrote:
>> On Thu, Mar 10, 2022 at 05:08:07PM +0100, Jan Beulich wrote:
>>> On 10.03.2022 16:47, Roger Pau Monné wrote:
>>>> On Thu, Mar 10, 2022 at 04:23:00PM +0100, Jan Beulich wrote:
>>>>> On 10.03.2022 15:34, Marek Marczykowski-Górecki wrote:
>>>>>> --- a/xen/drivers/char/ns16550.c
>>>>>> +++ b/xen/drivers/char/ns16550.c
>>>>>> @@ -1221,6 +1221,9 @@ pci_uart_config(struct ns16550 *uart, bool_t 
>>>>>> skip_amt, unsigned int idx)
>>>>>>                               pci_conf_read8(PCI_SBDF(0, b, d, f),
>>>>>>                                              PCI_INTERRUPT_LINE) : 0;
>>>>>>   
>>>>>> +                if (uart->irq >= nr_irqs)
>>>>>> +                    uart->irq = 0;
>>>>>
>>>>> Don't you mean nr_irqs_gsi here? Also (nit) please add the missing blanks
>>>>> immediately inside the parentheses.
>>>>
>>>> If we use nr_irqs_gsi we will need to make the check x86 only AFAICT.
>>>
>>> Down the road (when Arm wants to select HAS_PCI) - yes. Not necessarily
>>> right away. After all Arm wants to have an equivalent check here then,
>>> not merely checking against nr_irqs instead. So putting a conditional
>>> here right away would hide the need for putting in place an Arm-specific
>>> alternative.
>>
>> Oh, I always forget Arm doesn't have CONFIG_HAS_PCI enabled just yet.
> The PCI code in ns16550.c is gated by CONFIG_HAS_PCI and CONFIG_X86. I 
> am not sure we will ever see a support for PCI UART card in Xen on Arm.
> 
> However, if it evers happens then neither nr_irqs or nr_irqs_gsi would 
> help here because from the interrupt controller PoV 0xff may be a valid 
> (GICv2 supports up to 1024 interrupts).
> 
> Is there any reason we can't explicitely check 0xff?

FF isn't called out by the spec as having a special meaning. Unlike I
think Andrew did say somewhere, FF does not indicate "none". That's
instead indicated by PIN returning zero. That's my reading of the spec,
at least.

Jan