Re: [PATCH] x86: q35: require split irqchip for large CPU count

[Igor Mammedov]

On Fri, 11 Mar 2022 14:58:41 +0000
David Woodhouse <dwmw2@infradead.org> wrote:

> On Fri, 2022-03-11 at 09:39 -0500, Igor Mammedov wrote:
> > if VM is started with:
> > 
> >    -enable-kvm -smp 256
> > 
> > without specifying 'split' irqchip, VM might eventually boot
> > but no more than 255 CPUs will be operational and following
> > error messages in guest could be observed:
> >    ...
> >    smpboot: native_cpu_up: bad cpu 256
> >    ...
> > It's a regression introduced by [1], which removed dependency
> > on intremap=on that were implicitly requiring 'split' irqchip
> > and forgot to check for 'split' irqchip.
> > Instead of letting VM boot a broken VM, error out and tell
> > user how to fix CLI.  
> 
> Hm, wasn't that already fixed in the patches I posted in December?
It might be, could you point to the commit/series that fixed it.

Regardless of that, fixing it in recent kernels doesn't help
as still supported kernels are still affected by it.

If there is a way to detect that fix, I can add to q35 a compat
property and an extra logic to enable kernel-irqchip if fix is present.
Otherwise the fix does not exist until minimum supported kernel
version reaches version where it was fixed.