Re: [RFC PATCH 0/7] Pseudo-NMI for arm64 using ICC_PMR_EL1 (GICv3)

[Daniel Thompson]

On 01/04/15 16:29, Marc Zyngier wrote:
> On 01/04/15 16:15, Dave P Martin wrote:
> > Apologies for the slow reply... :/
> >
> > Anyway,
> >
> > On Mon, Mar 23, 2015 at 06:47:53PM +0000, Daniel Thompson wrote:
> > > On 20/03/15 15:45, Dave Martin wrote:
> > > > On Wed, Mar 18, 2015 at 02:20:21PM +0000, Daniel Thompson wrote:
> > > > > This patchset provides a pseudo-NMI for arm64 kernels by reimplementing
> > > > > the irqflags macros to modify the GIC PMR (the priority mask register is
> > > > > accessible as a system register on GICv3 and later) rather than the
> > > > > PSR. The pseudo-NMI changes are support by a prototype implementation of
> > > > > arch_trigger_all_cpu_backtrace that allows the new code to be exercised.
> >
> > Minor nit: the "pseudo NMI" terminology could lead to confusion if
> > something more closely resembling a real NMI comes along.
> >
> > I'll have to have a think, but nothing comes to mind right now...
> >
> > [...]
> >
> > > > >   3. Requires GICv3+ hardware together with firmware support to enable
> > > > >      GICv3 features at EL3. If CONFIG_USE_ICC_SYSREGS_FOR_IRQFLAGS is
> > > > >      enabled the kernel will not boot on older hardware. It will be hard
> > > > >      to diagnose because we will crash very early in the boot (i.e.
> > > > >      before the call to start_kernel). Auto-detection might be possible
> > > > >      but the performance and code size cost of adding conditional code to
> > > > >      the irqflags macros probably makes it impractical. As such it may
> > > > >      never be possible to remove this limitation (although it might be
> > > > >      possible to find a way to survive long enough to panic and show the
> > > > >      results on the console).
> > > >
> > > > This can (and should) be done via patching -- otherwise we risk breaking
> > > > single kernel image for GICv2+v3.
> > >
> > > Do you mean real patching (hunting down all those inlines and
> > > rewrite them) or simply implementing irqflags with an ops table? If
> > > the former I didn't look at this because I didn't release we could
> > > do that...
> >
> > A generic patching framework was introduced by Andre Przywara in this
> > patch:
> >
> > e039ee4 arm64: add alternative runtime patching
> >
> > I believe you should be able to use this to patch between DAIF and
> > ICC_PMR accesses.
> >
> > You should be able to find examples of this framework being used by
> > grepping.  I've not played with it myself yet.
>
> To follow-up on this, I have a few patches queued that use the runtime
> patching code to deal with GICv3 in KVM:
>
> http://thread.gmane.org/gmane.comp.emulators.kvm.arm.devel/616
>
> The first few patches are already queued for v4.1, and the rest should
> follow shortly after.

Thanks (both).

That's really helpful: links for that sort of thing are not easily 
googleable (things like kpatch floods the results).



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