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

[Daniel P . Berrangé]

On Mon, Mar 14, 2022 at 12:59:38PM +0000, David Woodhouse wrote:
> 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?

We've generally said QEMU should not reject / block startup of valid
hardware configurations, based on existance of bugs in certain guest
OS, if the config would be valid for other guest.

Regards,
Daniel
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