On Wed, 2022-03-16 at 05:56 -0400, Michael S. Tsirkin wrote: > On Wed, Mar 16, 2022 at 09:37:07AM +0000, David Woodhouse wrote: > > Yep, that's the guest operating system's choice. Not a qemu problem. > > > > Even if you have the split IRQ chip, if you boot a guest without kvm- > > msi-ext-dest-id support, it'll refuse to use higher CPUs. > > > > Or if you boot a guest without X2APIC support, it'll refuse to use > > higher CPUs. > > > > That doesn't mean a user should be *forbidden* from launching qemu in > > that configuration. > > Well the issue with all these configs which kind of work but not > the way they were specified is that down the road someone > creates a VM with this config and then expects us to maintain it > indefinitely. > > So yes, if we are not sure we can support something properly it is > better to validate and exit than create a VM guests don't know how > to treat. Not entirely sure how to reconcile that with what Daniel said in https://lore.kernel.org/qemu-devel/Yi9BTkZIM3iZsvdK@xxxxxxxxxx/ which was: > 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. That said, I cannot point at a *specific* example of a guest which can use the higher CPUs even when it can't direct external interrupts at them. I worked on making Linux capable of it, as I said, but didn't pursue that in the end. I *suspect* Windows might be able to do it, based on the way the hyperv-iommu works (by cheating and returning -EINVAL when external interrupts are directed at higher CPUs).
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