Re: [PATCH for-4.14 2/8] x86/hvm: don't force vCPU 0 for IRQ 0 when using fixed destination mode

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

On Thu, Jun 18, 2020 at 04:08:28PM +0200, Jan Beulich wrote:
> On 18.06.2020 15:48, Roger Pau Monné wrote:
> > On Thu, Jun 18, 2020 at 03:43:00PM +0200, Jan Beulich wrote:
> >> On 12.06.2020 17:56, Roger Pau Monne wrote:
> >>> When the IO APIC pin mapped to the ISA IRQ 0 has been configured to
> >>> use fixed delivery mode do not forcefully route interrupts to vCPU 0,
> >>> as the OS might have setup those interrupts to be injected to a
> >>> different vCPU, and injecting to vCPU 0 can cause the OS to miss such
> >>> interrupts or errors to happen due to unexpected vectors being
> >>> injected on vCPU 0.
> >>>
> >>> In order to fix remove such handling altogether for fixed destination
> >>> mode pins and just inject them according to the data setup in the
> >>> IO-APIC entry.
> >>>
> >>> Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
> >>
> >> Technically
> >> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
> >>
> >> I wonder though why this was done in the first place - it very much
> >> feels like a workaround for certain guest behavior, and hence
> >> getting rid of it may mean a certain risk of regressions. Not a
> >> very good point in time to make risky changes ...
> > 
> > We can defer to after the release I guess, but I will still ask for
> > the changes to be backported.
> 
> That's fine, albeit if we decide to delay it until 4.15 was branched,
> then I think we want to also wait longer than usual until it would hit
> the stable trees. Unfortunately c8e79412c001's description is of no
> help to understand what or why "time jumps" may result from delivering
> the interrupt as requested.

Yes, I've also looked at the original commit and have no idea what it
was actually trying to fix, and why delivering to vCPU 0 fixed it.
FWIW, I tried delivering to a different vCPU and it seems to work
fine.

Note that other timers (ie: RTC or HPET) are not tied to vCPU 0, so it
must have been something related to the PIT?

Thanks, Roger.