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

[Julien Grall <julien.grall@xxxxxxx>]

On 14/06/2019 15:36, Andrii Anisov wrote:
> Hello Julien,
Hi,

> On 13.06.19 17:41, Julien Grall wrote:
> > Sorry I may have missed it. We can't really restrict the usage of the current 
> > hypercall (it is pretty lax). So I think any lockless solution would require 
> > to allow the hypercall
> > to be used together (which I want to avoid).
> I'd better say here allowing using phys and virt registered runstates together 
> (and independently).
> And me personally for this approach, for sure not encouraging users (guests) to 
> do so.
Why? What are the benefits for a guest to use the two interface together? After 
all they have exactly the same data...
> > If we agree to allow the two hypercalls to be used together, then if we 
> > protect the update with domain_lock() then you should be able to avoid any 
> > race with the update path as onlining a vCPU requires to take the 
> > domain_lock() (see do_vcpu_op for x86 and do_common_cpu_on for Arm).
> Could you please clarify are you saying about protection runstate mapping update 
> or runstate values update?
runstate mapping.
> BTW, I'm a bit confused, are you OK with lock (not trylock) existing in 
> hypercall?
This is still in discussion.

Cheers,

--
Julien Grall

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