Re: [Xen-devel] [PATCH v3] xen/arm: introduce vwfi parameter

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 1 Mar 2017, Dario Faggioli wrote:
> On Tue, 2017-02-28 at 13:12 -0800, Stefano Stabellini wrote:
> > --- a/docs/misc/xen-command-line.markdown
> > +++ b/docs/misc/xen-command-line.markdown
> > @@ -1638,6 +1638,20 @@ Note that if **watchdog** option is also
> > specified vpmu will be turned off.
> >  As the virtualisation is not 100% safe, don't use the vpmu flag on
> >  production systems (see http://xenbits.xen.org/xsa/advisory-163.html
> > )!
> >  
> > +### vwfi
> > +> `= trap | native
> > +
> > +> Default: `trap`
> > +
> > +WFI is the ARM instruction to "wait for interrupt". WFE is similar
> > and
> > +means "wait for event". This option, which is ARM specific, changes
> > the
> > +way guest WFI and WFE are implemented in Xen. By default, Xen traps
> > both
> > +instructions. In the case of WFI, Xen blocks the guest vcpu; in the
> > case
> > +of WFE, Xen yield the guest vcpu. When setting vwfi to `native`, Xen
> > +doesn't trap either instruction, running them in guest context.
> > Setting
> > +vwfi to `native` reduces irq latency, but leads to suboptimal
> > scheduling
> > +decisions.
> > +
> These last few words sounds a bit generic to me. They may alarm people
> and discourage using 'native', even when it is actually ok.
> 
> Being a bit more specific, although without going into too much
> technical detail, seems worth to me.
> 
> So, how about something like:
> 
> "Using `native` reduces irq latency significantly. It can also lead to
> suboptimal scheduling decisions and affect performance, but only when
> the system is oversubscribed (i.e., there are, in total, more vCPUs
> than pCPUs)."

That's a good suggestion, thank you.

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