Re: [PATCH v2] x86/hvm: don't expose XENFEAT_hvm_pirqs by default

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Wed, Jan 10, 2024 at 01:42:02PM +0100, 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.
> 
> 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.
> 
> Disable HVM PIRQs by default and provide a per-domain option in xl.cfg to
> enable such feature.  Also for backwards compatibility keep the feature 
> enabled
> for any resumed domains that don't have an explicit selection.
> 
> Note that the only user of the feature (Linux) is also able to handle native
> interrupts fine, as the feature was already not used if Xen reported local 
> APIC
> hardware virtualization active.
> 
> Link: https://github.com/QubesOS/qubes-issues/issues/7971
> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

Should we have an entry in the changelog about this patch?

With the patch, we will need to regenerate the golang binding, at least
on commit.

Otherwise, patch looks fine:
Reviewed-by: Anthony PERARD <anthony.perard@xxxxxxxxxx>

Thanks,

-- 
Anthony PERARD