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

[Cédric Le Goater]

On 10/6/20 6:58 PM, Greg Kurz wrote:
> 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 ?

We use it in the lab on P9. We have never seen it failed even under stress. 
But there is a possible race in the logic. 

C.