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

[David Woodhouse <dwmw2@infradead.org>]

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On Mon, 2022-03-14 at 11:35 +0100, Igor Mammedov wrote:
> 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.

https://lore.kernel.org/all/20211209220840.14889-1-dwmw2@infradead.org/
is the patch I was thinking of, but although that moves the check to a
more useful place and fixes the X2APIC check, it *doesn't* include the
fix you're making; it's still using kvm_irqchip_in_kernel().

I can change that and repost the series, which is still sitting (with
fixed Reviewed-By/Acked-By attributions that I screwed up last time) in
https://git.infradead.org/users/dwmw2/qemu.git

> 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.

Hm, I'm not sure I follow here. Do you mean recent versions of *qemu*
when you say 'kernels'? 

I'm not even sure I agree with the observation that qemu should error
out here. The guest boots fine and the guest can even *use* all the
CPUs. IPIs etc. will all work fine. The only thing that doesn't work is
delivering *external* interrupts to CPUs above 254.

Ultimately, this is the *guest's* problem. Some operating systems can
cope; some can't.

The fact that *Linux* has a fundamental assumption that *all* CPUs can
receive all interrupts and that affinity can't be limited in hardware,
is a Linux problem. I tried to fix it once but it was distinctly non-
trivial and eventually I gave up and took a different approach.
https://lore.kernel.org/linux-iommu/87lfgj59mp.fsf@nanos.tec.linutronix.de/T/

But even if we 'fix' the check as you suggest to bail out and refuse to
boot a certain configuration because Linux guest wouldn't be able to
fully utilize it... Even if we boot with the split IRQ chip and the 15-
bit MSI enlightenment, we're still in the same position. Some guests
will be able to use it; some won't.

In fact, there are operating systems that don't even know about X2APIC.

Why should qemu refuse to even start up?

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