Re: [PATCH] Allow wake up of offline vcpu via nmi-ipi

[Keir Fraser <keir.xen@gmail.com>]

On 18/01/2012 09:31, "Keir Fraser" <keir.xen@gmail.com> wrote:

> On 18/01/2012 09:07, "Juergen Gross" <juergen.gross@ts.fujitsu.com> wrote:
> 
>> On 01/18/2012 09:48 AM, Juergen Gross wrote:
>>> On a real machine a cpu disabled via hlt with interrupts disabled can be
>>> reactivated via a nmi ipi. Enable the hypervisor to do this for hvm, too.
>>> 
>>> Signed-off-by: juergen.gross@ts.fujitsu.com
>>> 
>>> 
>>> 1 file changed, 4 insertions(+), 1 deletion(-)
>>> xen/arch/x86/hvm/vlapic.c |    5 ++++-
>> 
>> BTW: I was not able to reactivate a vcpu via INIT/SIPI/SIPI sequence. It
>> works
>> on initial system boot when the target vcpu is activated the first time. If I
>> deactivate a vcpu and try to activate it again it will start to run, but it
>> is
>> not starting at the specified entry point (at least it isn't performing the
>> first instruction there).
>> Is there some special initialization needed to make this work? Do I have to
>> reset
>> something on the vcpu before deactivating it?
> 
> No it should just work. Hvmloader wakes and then sleeps every AP, in
> hvmloader/smp.c. So even the first INIT-SIPI wakeup of an AP in the guest OS
> is not the first, as hvmloader already did it once! So this path should be
> working and indeed tested on every HVM guest boot.

Bit more info: INIT-SIPI logic is complicated by needing to avoid deadlocks
between two VCPUs attempting to pause and reset each other. But the core
dispatch logic is in vlapic_init_sipi_action(). You will see that on INIT,
we should call vcpu_reset() which will de-initialise and VCPU_down the vcpu.
And then on SIPI we call hvm_vcpu_reset_state(), which should reinitialise
and wake the vcpu to start running at the specified CS:IP.

So the above will be good places for you to add tracing and work out what's
going on. :-)

 -- Keir

>  -- Keir
> 
>> 
>> Juergen
> 
>