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Re: [Qemu-ppc] [PATCH 2/2] spapr: Adjust default VSMT value for better m

From: Greg Kurz
Subject: Re: [Qemu-ppc] [PATCH 2/2] spapr: Adjust default VSMT value for better migration compatibility
Date: Mon, 15 Jan 2018 10:48:47 +0100

On Mon, 15 Jan 2018 18:27:15 +1100
David Gibson <address@hidden> wrote:

> fa98fbfc "PC: KVM: Support machine option to set VSMT mode" introduced the
> "vsmt" parameter for the pseries machine type, which controls the spacing
> of the vcpu ids of thread 0 for each virtual core.  This was done to bring
> some consistency and stability to how that was done, while still allowing
> backwards compatibility for migration and otherwise.
> The default value we used for vsmt was set to the max of the host's
> advertised default number of threads and the number of vthreads per vcore
> in the guest.  This was done to continue running without extra parameters
> on older KVM versions which don't allow the VSMT value to be changed.
> Unfortunately, even that smaller than before leakage of host configuration
> into guest visible configuration still breaks things.  Specifically a guest
> with 4 (or less) vthread/vcore will get a different vsmt value when
> running on a POWER8 (vsmt==8) and POWER9 (vsmt==4) host.  That means the
> vcpu ids don't line up so you can't migrate between them, though you should
> be able to.
> Long term we really want to make vsmt == smp_threads for sufficiently
> new machine types.  However, that means that qemu will then require a
> sufficiently recent KVM (one which supports changing VSMT) - that's still
> not widely enough deployed to be really comfortable to do.
> In the meantime we some default that will work as often as possible.

s/we some/we need some/ ?

> This patch changes that default to 8 in all circumstances.  This does
> change guest visible behaviour (including for existing machine versions)
> for many cases - just not the most common/important case.
> Following is case by case justification for why this is still the least
> worst option.  Note that any of the old behaviours can still be duplicated
> after this patch, it's just that it requires manual intervention by
> setting the vsmt property on the command line.

IIUC this unconditionally breaks existing setups that rely on static
Micro-Threading on a POWER8 host (eg, subcores-per-core=2 on the host
and smp_threads=4). I have no evidence this is a widely used setup,
but FWIW it is documented in some IBM RedBooks:

"Performance Optimization and Tuning Techniques for IBM Power Systems
 Processors Including IBM POWER8"


"IBM PowerKVM: Configuration and Use"


Maybe the new behaviour could be added for new machine types only ?

Anyway, in case you don't want extra complexity,

Reviewed-by: Greg Kurz <address@hidden>

> KVM HV on POWER8 host:
>    This is the overwhelmingly common case in production setups, and is
>    unchanged by design.  POWER8 hosts will advertise a default VSMT mode
>    of 8, and > 8 vthreads/vcore isn't permitted
> KVM HV on POWER7 host:
>    Will break, but POWER7s allowing KVM were never released to the public.
> KVM HV on POWER9 host:
>    Not yet released to the public, breaking this now will reduce other
>    breakage later.
> KVM HV on PowerPC 970:
>    Will theoretically break it, but it was barely supported to begin with
>    and already required various user visible hacks to work.  Also so old
>    that I just don't care.
> TCG:
>    This is the nastiest one; it means migration of TCG guests (without
>    manual vsmt setting) will break.  Since TCG is rarely used in production
>    I think this is worth it for the other benefits.  It does also remove
>    one more barrier to TCG<->KVM migration which could be interesting for
>    debugging applications.
>    As with TCG, this will break migration of existing configurations,
>    without adding extra manual vsmt options.  As with TCG, it is rare in
>    production so I think the benefits outweigh breakages.
> Signed-off-by: David Gibson <address@hidden>
> ---
>  hw/ppc/spapr.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
> index e35214bfc3..8e5ef7c9de 100644
> --- a/hw/ppc/spapr.c
> +++ b/hw/ppc/spapr.c
> @@ -2305,9 +2305,14 @@ static void spapr_set_vsmt_mode(sPAPRMachineState 
> *spapr, Error **errp)
>          }
>          /* In this case, spapr->vsmt has been set by the command line */
>      } else {
> -        /* Choose a VSMT mode that may be higher than necessary but is
> -         * likely to be compatible with hosts that don't have VSMT. */
> -        spapr->vsmt = MAX(kvm_smt, smp_threads);
> +        /*
> +         * Default VSMT value is tricky, because we need it to be as
> +         * consistent as possible (for migration), but this requires
> +         * changing it for at least some existing cases.  We pick 8 as
> +         * the value that we'd get with KVM on POWER8, the
> +         * overwhelmingly common case in production systems.
> +         */
> +        spapr->vsmt = 8;
>      }
>      /* KVM: If necessary, set the SMT mode: */

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