Re: [PATCH v2 0/6] arm64: provide pseudo NMI with GICv3

[Joel Fernandes <joelaf@google.com>]

On Mon, Apr 30, 2018 at 2:46 AM Julien Thierry <julien.thierry@arm.com>
wrote:
[...]
> > On Wed, Jan 17, 2018 at 3:54 AM, Julien Thierry <julien.thierry@arm.com>
wrote:
> >> Hi,
> >>
> >> This series is a continuation of the work started by Daniel [1]. The
goal
> >> is to use GICv3 interrupt priorities to simulate an NMI.
> >>
> >> To achieve this, set two priorities, one for standard interrupts and
> >> another, higher priority, for NMIs. Whenever we want to disable
interrupts,
> >> we mask the standard priority instead so NMIs can still be raised. Some
> >> corner cases though still require to actually mask all interrupts
> >> effectively disabling the NMI.
> >>
> >> Of course, using priority masking instead of PSR.I comes at some cost.
On
> >> hackbench, the drop of performance seems to be >1% on average for this
> >> version. I can only attribute that to recent changes in the kernel as
> >
> > Do you have more specific performance data on the performance overhead
> > with this series?
> >

> Not at the moment. I was planning on doing a v3 anyway considering this
> series is getting a bit old and the GICv3 driver has had some
modifications.

Great! Looking forward to it, will try to find some time to review this set
as well.

> Once I get to it I can try to have more detailed performance data on a
> recent kernel. I've really only measured the performance on hackbench
> and on kernel build from defconfig (and for the kernel build the
> performance difference was completely hidden by the noise).

> >> hackbench seems slightly slower compared to my other benchmarks while
the
> >> runs with the use of GICv3 priorities have stayed in the same time
frames.
> >> KVM Guests do not seem to be affected preformance-wise by the host
using
> >> PMR to mask interrupts or not.
> >>
> >> Currently, only PPIs and SPIs can be set as NMIs. IPIs being currently
> >> hardcoded IRQ numbers, there isn't a generic interface to set SGIs as
NMI
> >> for now. I don't think there is any reason LPIs should be allowed to
be set
> >> as NMI as they do not have an active state.
> >> When an NMI is active on a CPU, no other NMI can be triggered on the
CPU.
> >>
> >>
> >> Requirements to use this:
> >> - Have GICv3
> >> - SCR_EL3.FIQ is set to 1 when linux runs
> >
> > Ah I see it mentioned here. Again, can you clarify if this is
> > something that can be misconfigured? Is it something that the
> > bootloader sets?
> >

> Yes, this is something that the bootloader sets and we have seen a few
> cases where it is set to 0, so it can be "misconfigured".

> It is not impossible to handle this case, but this bit affects the view
> the GICv3 CPU interface has on interrupt priority values. However it
> requires to add some conditions in both the interrupt handling and
> masking/unmasking code, so ideally we would avoid adding things to this.

> But the idea is that Linux only deals with group 1 interrupts, and group
> 1 interrupts are only signaled as FIQs when the execution state is
> secure or at EL3, which should never happen in Linux's case. So ideally
> we'd like firmwares to set up this bit properly rather than to have to
> deal with both cases when only one of them makes sense for Linux.

 From what I see, on all our platforms, FIQs are delivered to the secure
monitor only. Which is the reason for this patchset in the first place. I
can't imagine a usecase that is not designed like this (and have not come
across this), so its probably Ok to just assume SCR_EL3.FIQ is to 1.

In the future, if SCR_EL3.FIQ is set 0, then the NMI should use the FIQ
mechanism delivered to the non-secure OS.

Does what I say make sense or was I just shooting arrows in the dark? :-P

thanks,

- Joel