Re: [PATCH 1/4] target/i386: Fix sanity check on max APIC ID / X2APIC enablement

[David Woodhouse]

On Fri, 2022-03-18 at 15:17 +0100, Igor Mammedov wrote:
> On Thu, 17 Mar 2022 11:13:44 +0000 David Woodhouse <dwmw2@infradead.org> 
> wrote:
> > Thus far, I didn't think anyone had been talking about a dependency on
> > IOMMU. That doesn't make any sense at all. EXT_DEST_ID is perfectly
> > sufficient for Linux kernels from 5.10 onwards and they don't need the
> > IOMMU.
> 
> IOMMU was required before EXT_DEST_ID due to irq-remapping dependency,
> and that conservative config worked fine for both Linux and Windows
> guests. That's why I've raised question if we should revert restriction
> to the way it was back then.
> 
> With Linux pre-5.10 guests, dmesg output at least complains about
> IRQ remapping, so user has a small chance to be able to figure out
> that IOMMU should be configured to get all CPUs working.
> For post-5.10, all one gets is "bad cpu" without any clue as to why,
> if EXT_DEST_ID is not advertised by hypervisor.
>
> It would be better if guest kernel printed some error/warning in that case.

That case doesn't exist any more since your change. You *can't* launch
qemu with that many vCPUs and not support EXT_DEST_ID.

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