Re: [Xen-devel] XEN_DOMCTL_setvcpucontext and domain_pause()

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 08/04/2016 11:51 AM, Jan Beulich wrote:
>>>> On 04.08.16 at 10:21, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
>> Looking at xen/common/domctl.c, it appears that during handling of
>> XEN_DOMCTL_setvcpucontext, a domain_pause() happens unconditionally:
>>
>>  465         if ( ret == 0 )
>>  466         {
>>  467             domain_pause(d);
>>  468             ret = arch_set_info_guest(v, c);
>>  469             domain_unpause(d);
>>  470
>>  471             if ( ret == -ERESTART )
>>  472                 ret = hypercall_create_continuation(
>>  473                           __HYPERVISOR_domctl, "h", u_domctl);
>>  474         }
>>
>> I assume that this is because in xen/arch/x86/domain.c,
>> arch_set_info_guest() uses v->domain here:
> 
> Not only: It would be rather bad to change register state
> underneath a running vCPU (such a change then would not take
> effect right away, and might not [fully] take effect at all). Plus,
> if you paused only the subject vCPU, you'd risk races with other
> vCPU-s interacting with the paused one.
> 
> It's anyway questionable whether setting context for a vCPU
> after it got started is really such a good idea, even more so if
> you mean to do this frequently (and only then I can see that
> the pausing may get into the way).

We are indeed setting (mostly GPRs) somewhat frequently, for
already-paused VCPUs (paused because a sync-type vm_event has been sent,
and the responsible VCPU is frozen until we reply). In that case, we'd
like to be as efficient as possible (and assumed until recently that
that was the case when choosing xc_vcpu_setcontext() over
xc_domain_hvm_setcontext()).


Thanks,
Razvan


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