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

[Andrii Anisov <andrii.anisov@xxxxxxxxx>]

Hello Julien,

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.

> 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?
I'd not use the common lock for all vcpus runstate value update, unless it is 
an rw lock or doing trylock on it.

BTW, I'm a bit confused, are you OK with lock (not trylock) existing in 
hypercall?

--
Sincerely,
Andrii Anisov.

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