Re: [Xen-devel] [ARM] Native application design and discussion (I hope)

[Julien Grall <julien.grall@xxxxxxx>]

On 05/10/2017 10:56 AM, George Dunlap wrote:
> On 09/05/17 19:29, Stefano Stabellini wrote:
> > On Tue, 9 May 2017, Dario Faggioli wrote:
> > > > > And it should not be hard to give such code access to the context
> > > > > of
> > > > > the vCPU that was previously running (in x86, given we implement
> > > > > what
> > > > > we call lazy context switch, it's most likely still loaded in the
> > > > > pCPU!).
> > > > I agree with Stefano, switching to the idle vCPU is a pretty bad
> > > > idea.
> > > >
> > > > the idle vCPU is a fake vCPU on ARM to stick with the common code
> > > > (we
> > > > never leave the hypervisor). In the case of the EL0 app, we want to
> > > > change exception level to run the code with lower privilege.
> > > >
> > > > Also IHMO, it should only be used when there are nothing to run and
> > > > not
> > > > re-purposed for running EL0 app.
> > > It's already purposed for running when there is nothing to do _or_ when
> > > there are tasklets.
> > >
> > > I do see your point about privilege level, though. And I agree with
> > > George that it looks very similar to when, in the x86 world, we tried
> > > to put the infra together for switching to Ring3 to run some pieces of
> > > Xen code.
> > Right, and just to add to it, context switching to the idle vcpu has a
> > cost, but it doesn't give us any security benefits whatsever. If Xen is
> > going to spend time on context switching, it is better to do it in a
> > way that introduces a security boundary.
> "Context switching" to the idle vcpu doesn't actually save or change any
> registers, nor does it flush the TLB.  It's more or less just accounting
> for the scheduler.  So it has a cost (going through the scheduler) but
> not a very large one.
It depends on the architecture. For ARM we don't yet support lazy 
context switch. So effectively, the cost to "context switch" to the idle 
vCPU will be quite high.
Cheers,

--
Julien Grall

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