Re: More than 255 vcpus Windows VM setup without viommu ?

[David Woodhouse <dwmw2@infradead.org>]

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On Tue, 2024-07-02 at 05:17 +0000, Sandesh Patel wrote:
> Hi All,
> Is it possible to setup a large Windows VM (say 512 vcpus) without
> adding viommu (EIM=on, IR=on)?
> When I try to power such VM, the qemu process crashes with error-
> ```
> qemu-kvm: ../accel/kvm/kvm-all.c:1837: kvm_irqchip_commit_routes: Assertion `ret == 0’ failed
> 

Interesting. What exactly has Windows *done* in those MSI entries? That
might give a clue about how to support it.

> 
> The VM boots fine if we attach a vIOMMU but adding a vIOMMU can
> potentially result in IO performance loss in guest.
> I was interested to know if someone could boot a large Windows VM by
> some other means like kvm-msi-ext-dest-id.

I worked with Microsoft folks when I was defining the msi-ext-dest-id
support, and Hyper-V does it exactly the same way. But that's on the
*hypervisor* side. At the time, I don't believe Windows as a guest was
planning to use it. 

But I actually thought Windows worked OK without being able to direct
external interrupts to all vCPUs, so it didn't matter?


> Overheads of viommu have been shown for example in -
> https://static.sched.com/hosted_files/kvmforum2021/da/vIOMMU%20KVM%20
> Forum%202021%20-%20v4.pdf

Isn't that for DMA translation though? If you give the guest an
intel_iommu with dma_translation=off then it should *only* do interrupt
remapping.

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