Re: [Xen-devel] [PATCH] x86: move vgc_flags to struct pv_vcpu

[Jan Beulich <jbeulich@xxxxxxxx>]

On 03.01.2020 12:48, Julien Grall wrote:
> Hi Jan,
> 
> Thank you for the information.
> 
> On 03/01/2020 11:31, Jan Beulich wrote:
>> On 03.01.2020 12:19, Julien Grall wrote:
>>> How do you manage secondary CPUs on HVM/PVH guest?
>>
>> Secondary CPUs have architectural state they start with, and
>> there's very little control an OS has over initial register
>> state: There's just an 8-bit value specifying (part of) the
>> address the CPU should start executing from. All other
>> registers get set to hard coded values. And that 8-bit value
>> is part of the IPI message the primary CPU sends to the AP
>> to be brought up (i.e. there's no hypercall involved here).
> 
> Do you have any pointer to this code?

Perhaps best look at how Xen _uses_ this mechanism:
smpboot.c:wakeup_secondary_cpu().

> Can a CPU be turned off afterwards 
> and then boot again?

Yes, by sending it an INIT signal (as done by e.g.
wakeup_secondary_cpu()).

Jan

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