Re: Qemu problems in -next with 's390/spinlock: add niai spinlock hints'

[Cornelia Huck]

On Wed, 26 Jul 2017 07:40:44 +0200
Martin Schwidefsky <schwidefsky@xxxxxxxxxx> wrote:

> On Wed, 26 Jul 2017 07:00:33 +0200
> Heiko Carstens <heiko.carstens@xxxxxxxxxx> wrote:
> 
> > On Tue, Jul 25, 2017 at 06:14:51PM -0700, Guenter Roeck wrote:  
> > > Hi Martin,
> > > 
> > > my s390 qemu tests in linux-next stopped working a few days ago.
> > > Bisect points to commit 's390/spinlock: add niai spinlock hints'.
> > > 
> > > Looking at the patch, this isn't really surprising; at least to me it looks
> > > like the patch is making instructions mandatory which are only available in
> > > Z14 CPUs. Does this mean that older s390 CPUs (such as the Z900 used in my
> > > qemu tests) are no longer going to be supported in Linux ?    
> > 
> > No, that means that the patch has a bug. The NIAI instruction is only
> > available if the execution-hint facility is installed. That facility came
> > with zEC12. Luckily it uses the same facility indicator bit like the
> > miscellaneous-instruction-extensions facility, which we already use anyway
> > if the kernel gets compiled for zEC12. In that case we have early code
> > which verifies if all required facilities to run the kernel are installed,
> > and if not it will print a message to the console and stop the machine.
> > 
> > So the easiest fix would be to generate the NIAI instruction only if the
> > kernel gets compiled for zEC12 or newer.  
> 
> Hmm, I though that NIAI is a NOP on older machines.

FWIW, the upcoming qemu 2.11 (in tcg mode) will support to activate
stfle bit 49 and generate NOPs for NIAI and friends.

If I read the PoP correctly, the only correct way is to check for stfle
bit 49 before trying to use NIAI. While a real zEC12 or later probably
will have the facility, you can withdraw the feature bit via the cpu
model when running under qemu.