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

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Fri, 16 Feb 2018, Julien Grall wrote:
> 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.

Do you have a link to a doc somewhere that provides more details about
this? We could add a link to it here to inform users. It would be
useful.


> > 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.

People might know very well the hardware, and have very detailed
information about big.LITTLE and their platform, but might not know that
much about Xen.

I would like to introduce a tie between the warning message and the
documentation provided. The tie doesn't have to be around hmp-unsafe:
the intention wasn't really to provide a shortcut to do more damage, but
rather to inform about the situation. In fact, I didn't mean to
recommend the usage of hmp-unsafe.

Maybe we could say:

  See big.LITTLE under 
https://xenbits.xen.org/docs/unstable/misc/xen-command-line.html

or create docs/mics/arm/big.LITTLE ?

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