Re: [Qemu-devel] [PATCH 1/1] spapr: Check SMT based on KVM_CAP_PPC_SMT_POSSIBLE

[David Gibson <david@gibson.dropbear.id.au>]

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On Fri, Jan 05, 2018 at 10:47:22PM -0200, Jose Ricardo Ziviani wrote:
> Power9 supports 4 HW threads/core but it's possible to emulate
> doorbells to implement virtual SMT. KVM has the KVM_CAP_PPC_SMT_POSSIBLE
> which returns a bitmap with all SMT modes supported by the host.
> 
> Today, QEMU forces the SMT mode based on PVR compat table, this is
> silently done in spapr_fixup_cpu_dt. Then, if user passes thread=8 the
> guest will end up with 4 threads/core without any feedback to the user.
> It is confusing and will crash QEMU if a cpu is hotplugged in that
> guest.
> 
> This patch makes use of KVM_CAP_PPC_SMT_POSSIBLE to check if the host
> supports the SMT mode so it allows Power9 guests to have 8 threads/core
> if desired.
> 
> Reported-by: Satheesh Rajendran <satheera@in.ibm.com>
> Signed-off-by: Jose Ricardo Ziviani <joserz@linux.vnet.ibm.com>
> ---
>  hw/ppc/spapr.c       | 14 +++++++++++++-
>  hw/ppc/trace-events  |  1 +
>  target/ppc/kvm.c     |  5 +++++
>  target/ppc/kvm_ppc.h |  6 ++++++
>  4 files changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index d1acfe8858..ea2503cd2f 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -345,7 +345,19 @@ static int spapr_fixup_cpu_dt(void *fdt, sPAPRMachineState *spapr)
>          PowerPCCPU *cpu = POWERPC_CPU(cs);
>          DeviceClass *dc = DEVICE_GET_CLASS(cs);
>          int index = spapr_vcpu_id(cpu);
> -        int compat_smt = MIN(smp_threads, ppc_compat_max_threads(cpu));
> +
> +        /* set smt to maximum for this current pvr if the number
> +         * passed is higher than defined by PVR compat mode AND
> +         * if KVM cannot emulate it.*/
> +        int compat_smt = smp_threads;
> +        if ((kvmppc_cap_smt_possible() & smp_threads) != smp_threads &&
> +                smp_threads > ppc_compat_max_threads(cpu)) {
> +            compat_smt = ppc_compat_max_threads(cpu);

I don't think this is the right approach.  We've been trying to remove
places where host properties (such as those read from KVM
capabilities) affect guest visible properties of the VM - like vsmt.
Places like that break migration and often libvirt expectations as
well.

This is putting one back in, and so a step in the wrong direction.

-- 
David Gibson			| I'll have my music baroque, and my code
david AT gibson.dropbear.id.au	| minimalist, thank you.  NOT _the_ _other_
				| _way_ _around_!
http://www.ozlabs.org/~dgibson

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