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

[Jason Andryuk <jason.andryuk@amd.com>]

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@kernel.org> 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@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).
>>        > >
>>        > > 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