Xen project Mailing List

On Sat, Aug 17, 2024 at 2:45 AM Jason Andryuk <jason.andryuk@xxxxxxx> wrote:

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.

Thanks Stefano,

I'm going to send a v2 following this suggestion of using GUEST_MAX_VCPUS.

Will also add comments clarifying that this is a MAX value for the command-line option

and not what gets passed to register_ioreq.

We can continue the discussion from there to see if we want to change things,

I don't have a strong opinion here so I'm happy to go either way.

>
> 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.

Thanks Jason, yes, in that case the default of cpus=1, maxcpus=1 gets set. I was initially under the wrong assumption that without -smp options, the max would get set.

This is what I was trying to clarify in my previous email:

>> Looking at this a little more. If users (xl) don't pass an -smp option we actually default to smp.max_cpus=1.

Best regards,

Edgar

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