Re: [Qemu-devel] [Qemu-ppc] [PATCH v3 0/4] target-ppc: Add FWNMI support in qemu for powerKVM guests

[Sam Bobroff <sam.bobroff@au1.ibm.com>]

On Thu, Sep 03, 2015 at 09:53:20AM +1000, David Gibson wrote:
> On Wed, Sep 02, 2015 at 04:34:01PM +1000, Sam Bobroff wrote:
> > On Tue, Sep 01, 2015 at 04:37:51PM +0530, Aravinda Prasad wrote:
> > > 
> > > 
> > > On Monday 10 August 2015 09:35 AM, Sam Bobroff wrote:
> > > > On Sun, Aug 09, 2015 at 03:53:02PM +0200, Alexander Graf wrote:
> > > >>
> > > >>
> > > >> On 07.08.15 05:37, Sam Bobroff wrote:
> [snip]
> > > >>> (c) Assemble it (as above) but include it directly in the QEMU binary by
> > > >>> objcopying it in or hexdumping into a C string or something similar. This seems
> > > >>> fairly neat but I'm not sure how people would feel about including "binaries"
> > > >>> into QEMU this way.  Although it would take some work in the build system, it
> > > >>> seems like a fairly neat solution to me.
> > > >>
> > > >> We tried to move away from code as hex arrays in QEMU to make it easier
> > > >> for people to patch things when they want to. But then again if we're
> > > >> talking 3 instructions it might not be the worst option.
> > > > 
> > > > Sounds sensible.
> > > > 
> > > > So, in summary, it sounds like a decent approach would be:
> > > > * store the guest's handlers in QEMU's spapr structure,
> > > > * simplify the trampolines down to a single, non-returning, hcall,
> > > 
> > > However, other instructions such as saving r3 and re-trying hcall are
> > > still required.
> > 
> > Ah yes, that's true. I was thinking that the retrying could happen inside the
> > hcall but it can't.
> 
> Sorry, I may have missed something here.  What does the code in the
> vector need to retry?

It's due to having to handle simtaneous machine checks and there being a single
shared buffer for reporting the error. PAPR isn't very specific but here is
what it says (from section 7.3.14):

Multiple processors of the same OS image may experi- ence fatal events at, or
about, the same time. The first processor to enter the machine check handling
firmware reports the fatal error. Subsequent processors serialize waiting for
the first processor to issue the ibm,nmi-interlock call. These subsequent
processors report “fatal error previously reported”. If, after the firmware
makes a Machine Check call back, and before the OS issues the ibm,nmi-interlock
call, the same processor that is currently holding the storage containing the
error log structure receives another Machine Check NMI, the firmware has no
choice but to declare the condition fatal, log the result and execute the
partition’s reboot policy.

So it needs to retry setting up the error buffer until it succeeds.

> Also, it looks like the vector will need at least one scratch register
> (for the hcall number, if nothing else).  Does PAPR specify what SPRGs
> the vector can clobber?  Obviously it can't be anything the guest
> kernel uses.

PAPR only says SPRGs 0 to 3 are for software use, but the kernel (see
arch/powerpc/include/asm/reg.h) defines SPRG2 as an exception scratch register
so it should be the right one to use here.

> Btw, does anyone know what happens with the VPA (and dispatch trace
> log and so forth) on kexec() - it could be subject to the same stale
> address problem, and rewriting vectors won't save us there.

I asked Michael Ellerman this one and he thinks kexec probably frees and
re-allocates the VPA.

Sam.