Re: [PATCH v2 3/6] spapr/xive: Add a warning when StoreEOI is activated on POWER9 CPUs

[Greg Kurz]

On Mon, 5 Oct 2020 18:51:44 +0200
Cédric Le Goater <clg@kaod.org> wrote:

> StoreEOI on POWER9 CPUs is racy because load-after-store ordering is
> not enforced.
> 
> Signed-off-by: Cédric Le Goater <clg@kaod.org>
> ---
>  hw/ppc/spapr_caps.c | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/hw/ppc/spapr_caps.c b/hw/ppc/spapr_caps.c
> index b0a9d0227db2..9251badbdc27 100644
> --- a/hw/ppc/spapr_caps.c
> +++ b/hw/ppc/spapr_caps.c
> @@ -549,6 +549,15 @@ static void cap_storeeoi_apply(SpaprMachineState *spapr, 
> uint8_t val,
>              error_setg(errp, "StoreEOI not supported by KVM");
>              return;
>          }
> +
> +        /*
> +         * load-after-store ordering is not enforced on POWER9 CPUs
> +         * and StoreEOI can be racy.
> +         */
> +        if (!ppc_type_check_compat(machine->cpu_type, 
> CPU_POWERPC_LOGICAL_3_10,
> +                                  0, spapr->max_compat_pvr)) {
> +            warn_report("StoreEOI on a POWER9 CPU is unsafe on KVM.");

It all boils down to what "unsafe" really means here... if the outcome is
"very likely hang the guest" as soon as it starts doing I/O, shouldn't
we error out instead ? What is the motivation to use StoreEOI if the
processor doesn't really support it ?

> +        }
>      }
>  }
>