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

[Julien Grall <julien@xxxxxxx>]

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? Can a CPU be turned off afterwards 
and then boot again?
> For PVH, a variant of the normal PV model of starting vCPU-s
> gets used, i.e. via VCPUOP_initialise.
In the case of PSCI, I think it is between the two. We are using a 
generic hypercall, yet most of the state is fixed.
But as the guest OS may run a CPU for a while, turning off and then boot 
again, we need to be able to set the state again. Hence, the 
arch_set_guest_info() is quite convenient to use to reset the CPU state.
Cheers,

--
Julien Grall

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