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

[Jason Andryuk <jason.andryuk@xxxxxxx>]

On 2024-08-16 12:53, Stefano Stabellini wrote:
> On Fri, 16 Aug 2024, Edgar E. Iglesias wrote:
> > On Thu, Aug 15, 2024 at 2:30 AM Stefano Stabellini <sstabellini@xxxxxxxxxx> 
> > wrote:
> >        On Wed, 14 Aug 2024, Edgar E. Iglesias wrote:
> >        > On Tue, Aug 13, 2024 at 03:52:32PM -0700, Stefano Stabellini wrote:
> >        > > On Tue, 13 Aug 2024, Edgar E. Iglesias wrote:
> >        > > > 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@xxxxxxx>
> >        > > > > >
> >        > > > > > Add SMP support for Xen PVH ARM guests. Create max_cpus ioreq
> >        > > > > > servers to handle hotplug.
> >        > > > > >
> >        > > > > > Signed-off-by: Edgar E. Iglesias <edgar.iglesias@xxxxxxx>
> >        > > > > > ---
> >        > > > > >  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).
> >        > >
> >        > > What do you mean by "non-pvh aware xl"? All useful versions of xl
> >        > > support pvh?
> >        >
> >        >
> >        > I mean an xl without our PVH patches merged.
> >        > xl in upstream doesn't know much about PVH yet.
> >        > Even for ARM, we're still carrying significant patches in our tree.
> >
> >        Oh I see. In that case, I don't think we need to support "non-pvh aware 
> > xl".
> >
> >
> >        > > > > 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.
> >        > >
> >        > > OK good. In that case if this is just an initial value meant to be
> >        > > overwritten, I think it is best to keep it as 1.
> >        >
> >        > Sorry but that won't work. I think the confusion here may be that
> >        > it's easy to mix up mc->max_cpus and machine->smp.max_cpus, these are
> >        > not the same. They have different purposes.
> >        >
> >        > I'll try to clarify the 3 values in play.
> >        >
> >        > machine-smp.cpus:
> >        > Number of guest vcpus active at boot.
> >        > Passed to QEMU via the -smp command-line option.
> >        > We don't use this value in QEMU's ARM PVH machines.
> >        >
> >        > machine->smp.max_cpus:
> >        > Max number of vcpus that the guest can use (equal or larger than 
> > machine-smp.cpus).
> >        > Will be set by xl via the "-smp X,maxcpus=Y" command-line option to 
> > QEMU.
> >        > Taken from maxvcpus from xl.cfg, same as XEN_DMOP_nr_vcpus.
> >        > This is what we use for xen_register_ioreq().
> >        >
> >        > mc->max_cpus:
> >        > Absolute MAX in QEMU used to cap the -smp command-line options.
> >        > If xl tries to set -smp (machine->smp.max_cpus) larger than this, QEMU 
> > will bail out.
> >        > Used to setup xen_register_ioreq() ONLY if -smp maxcpus was NOT set 
> > (i.e by a non PVH aware xl).
> >        > Cannot be 1 because that would limit QEMU to MAX 1 vcpu.
> >        >
> >        > I guess we could set mc->max_cpus to what XEN_DMOP_nr_vcpus returns 
> > but I'll
> >        > have to check if we can even issue that hypercall this early in QEMU 
> > since
> >        > mc->max_cpus is setup before we even parse the machine options. We may
> >        > not yet know what domid we're attaching to yet.
> >
> >        If mc->max_cpus is the absolute max and it will not be used if -smp is
> >        passed to QEMU, then I think it is OK to use GUEST_MAX_VCPUS
> >
> > Looking at this a little more. If users (xl) don't pass an -smp option we 
> > actually default to smp.max_cpus=1.
> > So, another option is to simply remove the upper limit in QEMU (e.g we can set 
> > mc->max_cpus to something very large like UINT32_MAX).
> > That would avoid early hypercalls, avoid using GUEST_MAX_VCPUS and always let 
> > xl dictate the max_cpus value using the -smp cmdline option.
> As the expectation is that there will be always a smp.max_cpus option
> passed to QEMU, I would avoid an extra early hypercall.
>
> For the initial value, I would use something static and large, but not
> unreasonably large as UINT32_MAX to be more resilient in (erroneous)
> cases where smp.max_cpus is not passed.
>
> So I would initialize it to GUEST_MAX_VCPUS, or if we don't want to use
> GUEST_MAX_VCPUS, something equivalent in the 64-256 range.
>
> Alternative we can have a runtime check and exit with a warning if
> smp.max_cpus is not set.
FYI, xl only passes a -smp option when the domU has more than 1 vcpu. 
Though that implies only a single vcpu.
Regards,
Jason