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Re: [PATCH] x86/hvm: don't expose XENFEAT_hvm_pirqs by default
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Wed, 10 Jan 2024 11:26:02 +0100
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- Cc: Wei Liu <wl@xxxxxxx>, Anthony PERARD <anthony.perard@xxxxxxxxxx>, Juergen Gross <jgross@xxxxxxxx>, Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Wed, 10 Jan 2024 10:26:12 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 10.01.2024 10:53, Roger Pau Monne wrote:
> The HVM pirq feature allows routing interrupts from both physical and emulated
> devices over event channels, this was done a performance improvement. However
> its usage is fully undocumented, and the only reference implementation is in
> Linux. It defeats the purpose of local APIC hardware virtualization, because
> when using it interrupts avoid the usage of the local APIC altogether.
So without sufficient APIC acceleration, isn't this arranging for degraded
performance then? IOW should the new default perhaps be dependent on the
degree of APIC acceleration?
> It has also been reported to not work properly with certain devices, at least
> when using some AMD GPUs Linux attempts to route interrupts over event
> channels, but Xen doesn't correctly detect such routing, which leads to the
> hypervisor complaining with:
>
> (XEN) d15v0: Unsupported MSI delivery mode 7 for Dom15
>
> When MSIs are attempted to be routed over event channels the entry delivery
> mode is set to ExtINT, but Xen doesn't detect such routing and attempts to
> inject the interrupt following the native MSI path, and the ExtINT delivery
> mode is not supported.
Shouldn't this be properly addressed nevertheless? The way it's described
it sounds as if MSI wouldn't work at all this way; I can't spot why the
issue would only be "with certain devices". Yet that in turn doesn't look
to be very likely - pass-through use cases, in particular SR-IOV ones,
would certainly have noticed.
Jan
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