Re: [Xen-devel] [PATCH v3] xen: introduce VCPUOP_register_runstate_phys_memory_area hypercall

[Andrii Anisov <andrii.anisov@xxxxxxxxx>]

Jan, Julien,

On 11.06.19 12:10, Jan Beulich wrote:
> > > At the very least such loops want a cpu_relax() in their bodies.
> > > But this being on a hypercall path - are there theoretical guarantees
> > > that a guest can't abuse this to lock up a CPU?
> > Hmmm, I suggested this but it looks like a guest may call the hypercall multiple
> > time from different vCPU. So this could be a way to delay work on the CPU.
> >
> > I wanted to make the context switch mostly lockless and therefore avoiding to
> > introduce a spinlock.
> Well, constructs like the above are trying to mimic a spinlock
> without actually using a spinlock. There are extremely rare
> situation in which this may indeed be warranted, but here it
> falls in the common "makes things worse overall" bucket, I
> think. To not unduly penalize the actual update paths, I think
> using a r/w lock would be appropriate here.
So what is the conclusion here? Should we go with trylock and 
hypercall_create_continuation() in order to avoid locking but still not fail to 
the guest?

--
Sincerely,
Andrii Anisov.

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