Re: [PATCH v1 04/10] hw/arm: xenpvh: Add support for SMP guests

["Edgar E. Iglesias" <edgar.iglesias@gmail.com>]

On Mon, Aug 12, 2024 at 06:47:17PM -0700, Stefano Stabellini wrote:
> On Mon, 12 Aug 2024, Edgar E. Iglesias wrote:
> > From: "Edgar E. Iglesias" <edgar.iglesias@amd.com>
> > 
> > Add SMP support for Xen PVH ARM guests. Create max_cpus ioreq
> > servers to handle hotplug.
> > 
> > Signed-off-by: Edgar E. Iglesias <edgar.iglesias@amd.com>
> > ---
> >  hw/arm/xen_arm.c | 5 +++--
> >  1 file changed, 3 insertions(+), 2 deletions(-)
> > 
> > diff --git a/hw/arm/xen_arm.c b/hw/arm/xen_arm.c
> > index 5f75cc3779..ef8315969c 100644
> > --- a/hw/arm/xen_arm.c
> > +++ b/hw/arm/xen_arm.c
> > @@ -173,7 +173,7 @@ static void xen_arm_init(MachineState *machine)
> >  
> >      xen_init_ram(machine);
> >  
> > -    xen_register_ioreq(xam->state, machine->smp.cpus, &xen_memory_listener);
> > +    xen_register_ioreq(xam->state, machine->smp.max_cpus, &xen_memory_listener);
> >  
> >      xen_create_virtio_mmio_devices(xam);
> >  
> > @@ -218,7 +218,8 @@ static void xen_arm_machine_class_init(ObjectClass *oc, void *data)
> >      MachineClass *mc = MACHINE_CLASS(oc);
> >      mc->desc = "Xen PVH ARM machine";
> >      mc->init = xen_arm_init;
> > -    mc->max_cpus = 1;
> > +    /* MAX number of vcpus supported by Xen.  */
> > +    mc->max_cpus = GUEST_MAX_VCPUS;
> 
> Will this cause allocations of data structures with 128 elements?
> Looking at hw/xen/xen-hvm-common.c:xen_do_ioreq_register it seems
> possible? Or hw/xen/xen-hvm-common.c:xen_do_ioreq_register is called

Yes, in theory there's probably overhead with this but as you correctly
noted below, a PVH aware xl will set the max_cpus option to a lower value.

With a non-pvh aware xl, I was a little worried about the overhead
but I couldn't see any visible slow-down on ARM neither in boot or in network
performance (I didn't run very sophisticated benchmarks).


> later on with the precise vCPU value which should be provided to QEMU
> via the -smp command line option
> (tools/libs/light/libxl_dm.c:libxl__build_device_model_args_new)?

Yes, a pvh aware xl will for example pass -smp 2,maxcpus=4 based on
values from the xl.cfg. If the user doesn't set maxvcpus in xl.cfg, xl
will set maxvcpus to the same value as vcpus.

Best regards,
Edgar