Re: sched=null vwfi=native and call_rcu()

[Julien Grall <julien@xxxxxxx>]

Hi,

On 17/01/2022 15:13, Juergen Gross wrote:
> On 17.01.22 12:05, George Dunlap wrote:
> > > On Jan 14, 2022, at 9:01 PM, Stefano Stabellini 
> > > <sstabellini@xxxxxxxxxx> wrote:
> > > On Fri, 14 Jan 2022, Dario Faggioli wrote:
> > > > On Thu, 2022-01-06 at 17:52 -0800, Stefano Stabellini wrote:
> > > > > On Thu, 6 Jan 2022, Julien Grall wrote:
> > > > Alternatively, we can submit the series as ARM-only... But I fear that
> > > > the x86 side of things would then be easily forgotten. :-(
> > > I agree with you on this, but at the same time we are having problems
> > > with customers in the field -- it is not like we can wait to solve the
> > > problem on ARM any longer. And the issue is certainly far less likely to
> > > happen on x86 (there is no vwfi=native, right?) In other words, I think
> > > it is better to have half of the solution now to solve the worst part of
> > > the problem than to wait more months for a full solution.
Well, it all depends on how your guest OS works A "malicious" guest that 
will configure the vCPU to busy loop without wfi will result to the same 
problem (this is one of the reason why NULL scheduler is not security 
supported).
> > An x86 equivalent of vwfi=native could be implemented easily, but 
> > AFAIK nobody has asked for it yet.  I agree that we need to fix if for 
> > ARM, and so in the absence of someone with the time to fix up the x86 
> > side, I think fixing ARM-only is the way to go.
> > It would be good if we could add appropriate comments warning anyone 
> > who implements `hlt=native` on x86 the problems they’ll face and how 
> > to fix them.  Not sure the best place to do that; in the VMX / SVM 
> > code that sets the exit for HLT &c?
> But wouldn't a guest in a busy loop on x86 with NULL scheduler suffer
> from the same problem?
This is not a problem on x86 because there will always an exit to the 
hypervisor a timed interval (IIRC for some rendezvous?). On Arm, using 
the NULL scheduler will result to a completely tickless hypervisor.
Cheers,

--
Julien Grall