Re: [PATCH] x86/IRQ: bump max number of guests for a shared IRQ to 31

[Jan Beulich <jbeulich@suse.com>]

On 01.12.2020 00:59, Igor Druzhinin wrote:
> Current limit of 7 is too restrictive for modern systems where one GSI
> could be shared by potentially many PCI INTx sources where each of them
> corresponds to a device passed through to its own guest. Some systems do not
> apply due dilligence in swizzling INTx links in case e.g. INTA is declared as
> interrupt pin for the majority of PCI devices behind a single router,
> resulting in overuse of a GSI.
> 
> Signed-off-by: Igor Druzhinin <igor.druzhinin@citrix.com>
> ---
> 
> If people think that would make sense - I can rework the array to a list of
> domain pointers to avoid the limit.

Not sure about this. What is the biggest number you've found on any
system? (I assume the chosen value of 31 has some headroom.)

Instead I'm wondering whether this wouldn't better be a Kconfig
setting (or even command line controllable). There don't look to be
any restrictions on the precise value chosen (i.e. 2**n-1 like is
the case for old and new values here, for whatever reason), so a
simple permitted range of like 4...64 would seem fine to specify.
Whether the default then would want to be 8 (close to the current
7) or higher (around the actually observed maximum) is a different
question.

Jan