Re: [PATCH] x86/irq: do not insert IRQ_MSI_EMU in emuirq mappings

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Oct 31, 2023 at 03:30:37PM +0200, Xenia Ragiadakou wrote:
> Do not use emuirq mappings for MSIs injected by emulated devices.
> This kind of pirq shares the same emuirq value and is not remapped.

AFAICT adding the extra emuirq mappings is harmless, and just adds
an extra layer of translation?

Or is this causing issues, but we haven't realized because we don't
provide emulated devices that use MSI(-X) by default?

> Fixes: 88fccdd11ca0 ('xen: event channel remapping for emulated MSIs')
> Signed-off-by: Xenia Ragiadakou <xenia.ragiadakou@xxxxxxx>
> ---
> 
> Question: is there any strong reason why Linux HVM guests still use pirqs?

Baggage I guess.  I've suggested in the past to switch PIRQs off by
default for HVM, but I had no figures to show how much of a
performance penalty that would be for passthrough devices.

My suggestion would be to introduce an xl.cfg option to select the
availability of PIRQs for HVM guests, and set it to off by default.
You would also need to make sure that migration (or save/restore)
works fine, and that incoming guests from previous Xen versions (that
won't have the option) will result in PIRQs still being enabled.

There's already a XEN_X86_EMU_USE_PIRQ flag in xen_arch_domainconfig,
so you just need to wire the tools side in order to allow selection by
users.

> 
>  xen/arch/x86/irq.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/xen/arch/x86/irq.c b/xen/arch/x86/irq.c
> index f42ad539dc..cdc8dc5a55 100644
> --- a/xen/arch/x86/irq.c
> +++ b/xen/arch/x86/irq.c
> @@ -2684,7 +2684,7 @@ int map_domain_emuirq_pirq(struct domain *d, int pirq, 
> int emuirq)
>      }
>  
>      old_emuirq = domain_pirq_to_emuirq(d, pirq);
> -    if ( emuirq != IRQ_PT )
> +    if ( (emuirq != IRQ_PT) && (emuirq != IRQ_MSI_EMU) )
>          old_pirq = domain_emuirq_to_pirq(d, emuirq);

I think you can just use emuirq >= 0, as we then only need the emuirq
translation for passthrough interrupts, same for the rest of the
changed conditions.

Looking further, the function seems to be useless when called with
emuirq < 0, and hence it might be better to avoid such calls in the
first place?

I have to admit I've always been very confused with the PIRQ logic, so
it's possible I'm missing some relevant stuff here.

Thanks, Roger.