Re: [Xen-devel] [PATCH 4/4] xen/arm: update the docs about heterogeneous computing

[Julien Grall <julien.grall@xxxxxxx>]

On 16/02/2018 20:50, Stefano Stabellini wrote:
> On Fri, 16 Feb 2018, Julien Grall wrote:
> > Hi Stefano,
> >
> > On 15/02/18 23:17, Stefano Stabellini wrote:
> > > Update the documentation of the hmp-unsafe option to explain how to use
> > > it safely, together with the right cpu affinity setting, on big.LITTLE
> > > systems.
> > >
> > > Also update the warning message to point users to the docs.
> > >
> > > Signed-off-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>
> > > CC: jbeulich@xxxxxxxx
> > > CC: konrad.wilk@xxxxxxxxxx
> > > CC: tim@xxxxxxx
> > > CC: wei.liu2@xxxxxxxxxx
> > > CC: andrew.cooper3@xxxxxxxxxx
> > > CC: George.Dunlap@xxxxxxxxxxxxx
> > > CC: ian.jackson@xxxxxxxxxxxxx
> > >
> > > ---
> > >    docs/misc/xen-command-line.markdown | 10 +++++++++-
> > >    xen/arch/arm/smpboot.c              |  9 +++++----
> > >    2 files changed, 14 insertions(+), 5 deletions(-)
> > >
> > > diff --git a/docs/misc/xen-command-line.markdown
> > > b/docs/misc/xen-command-line.markdown
> > > index 2184cb9..a1ebeea 100644
> > > --- a/docs/misc/xen-command-line.markdown
> > > +++ b/docs/misc/xen-command-line.markdown
> > > @@ -1007,7 +1007,15 @@ Control Xens use of the APEI Hardware Error Source
> > > Table, should one be found.
> > >      Say yes at your own risk if you want to enable heterogenous computing
> > >    (such as big.LITTLE). This may result to an unstable and insecure
> > > -platform. When the option is disabled (default), CPUs that are not
> > > +platform, unless you manually specify the cpu affinity of all domains so
> > > +that all vcpus are scheduled on the same class of pcpus (big or LITTLE
> > > +but not both). vcpu migration between big cores and LITTLE cores is not
> > > +supported. Thus, if the first 4 pcpus are big and the last 4 are LITTLE,
> > > +all domains need to have either cpus = "0-3" or cpus = "4-7" in their VM
> > > +config. Moreover, dom0_vcpus_pin needs to be passed on the Xen command
> > > +line.
> > In your example here you suggest to have all the vCPUs of a guest to either on
> > big or LITTLE cores. How about giving an example where the guest can have 2
> > LITTLE vCPUs and one big vCPU?
> I would rather discourage it at the moment, given that it requires more
> complex cpu affinity settings, or vcpu pinning. Also, I am afraid that
> without matching corresponding topology information on the guest device
> tree, guests might not work as expected in such a scenario.
>
> What do you think?
You already know my view on this. I would rather strongly discourage 
anyone pinning all vCPUs of a domain to big cores. We should avoid to 
provide shortcuts to use that could have potentially damageable impact 
on their platform without telling them.
> I see. What about:
>
>    See docs/misc/xen-command-line.markdown:hmp-unsafe
>
> Or:
>
>    See hmp-unsafe under 
> https://xenbits.xen.org/docs/unstable/misc/xen-command-line.html
I believe that people looking at big.LITTLE (and really want it) are 
smart enough to look at the docs or the code themselves. Given how 
fragile is your solution, I would rather avoid to help people doing bad 
thing.
Cheers,

--
Julien Grall

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