Re: [Qemu-devel] [PATCH v10 2/6] ppc: spapr: Introduce FWNMI capability

[Aravinda Prasad <aravinda@linux.vnet.ibm.com>]

On Friday 05 July 2019 06:37 AM, David Gibson wrote:
> On Thu, Jul 04, 2019 at 10:33:11AM +0530, Aravinda Prasad wrote:
>>
>>
>> On Thursday 04 July 2019 06:37 AM, David Gibson wrote:
>>> On Wed, Jul 03, 2019 at 02:58:24PM +0530, Aravinda Prasad wrote:
>>>>
>>>>
>>>> On Wednesday 03 July 2019 08:33 AM, David Gibson wrote:
>>>>> On Tue, Jul 02, 2019 at 11:54:26AM +0530, Aravinda Prasad wrote:
>>>>>>
>>>>>>
>>>>>> On Tuesday 02 July 2019 09:21 AM, David Gibson wrote:
>>>>>>> On Wed, Jun 12, 2019 at 02:51:04PM +0530, Aravinda Prasad wrote:
>>>>>>>> Introduce the KVM capability KVM_CAP_PPC_FWNMI so that
>>>>>>>> the KVM causes guest exit with NMI as exit reason
>>>>>>>> when it encounters a machine check exception on the
>>>>>>>> address belonging to a guest. Without this capability
>>>>>>>> enabled, KVM redirects machine check exceptions to
>>>>>>>> guest's 0x200 vector.
>>>>>>>>
>>>>>>>> This patch also introduces fwnmi-mce capability to
>>>>>>>> deal with the case when a guest with the
>>>>>>>> KVM_CAP_PPC_FWNMI capability enabled is attempted
>>>>>>>> to migrate to a host that does not support this
>>>>>>>> capability.
>>>>>>>>
>>>>>>>> Signed-off-by: Aravinda Prasad <aravinda@linux.vnet.ibm.com>
>>>>>>>> ---
>>>>>>>>  hw/ppc/spapr.c         |    1 +
>>>>>>>>  hw/ppc/spapr_caps.c    |   26 ++++++++++++++++++++++++++
>>>>>>>>  include/hw/ppc/spapr.h |    4 +++-
>>>>>>>>  target/ppc/kvm.c       |   19 +++++++++++++++++++
>>>>>>>>  target/ppc/kvm_ppc.h   |   12 ++++++++++++
>>>>>>>>  5 files changed, 61 insertions(+), 1 deletion(-)
>>>>>>>>
>>>>>>>> diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
>>>>>>>> index 6dd8aaa..2ef86aa 100644
>>>>>>>> --- a/hw/ppc/spapr.c
>>>>>>>> +++ b/hw/ppc/spapr.c
>>>>>>>> @@ -4360,6 +4360,7 @@ static void spapr_machine_class_init(ObjectClass *oc, void *data)
>>>>>>>>      smc->default_caps.caps[SPAPR_CAP_NESTED_KVM_HV] = SPAPR_CAP_OFF;
>>>>>>>>      smc->default_caps.caps[SPAPR_CAP_LARGE_DECREMENTER] = SPAPR_CAP_ON;
>>>>>>>>      smc->default_caps.caps[SPAPR_CAP_CCF_ASSIST] = SPAPR_CAP_OFF;
>>>>>>>> +    smc->default_caps.caps[SPAPR_CAP_FWNMI_MCE] = SPAPR_CAP_OFF;
>>>>>>>>      spapr_caps_add_properties(smc, &error_abort);
>>>>>>>>      smc->irq = &spapr_irq_dual;
>>>>>>>>      smc->dr_phb_enabled = true;
>>>>>>>> diff --git a/hw/ppc/spapr_caps.c b/hw/ppc/spapr_caps.c
>>>>>>>> index 31b4661..2e92eb6 100644
>>>>>>>> --- a/hw/ppc/spapr_caps.c
>>>>>>>> +++ b/hw/ppc/spapr_caps.c
>>>>>>>> @@ -479,6 +479,22 @@ static void cap_ccf_assist_apply(SpaprMachineState *spapr, uint8_t val,
>>>>>>>>      }
>>>>>>>>  }
>>>>>>>>  
>>>>>>>> +static void cap_fwnmi_mce_apply(SpaprMachineState *spapr, uint8_t val,
>>>>>>>> +                                Error **errp)
>>>>>>>> +{
>>>>>>>> +    if (!val) {
>>>>>>>> +        return; /* Disabled by default */
>>>>>>>> +    }
>>>>>>>> +
>>>>>>>> +    if (tcg_enabled()) {
>>>>>>>> +        error_setg(errp,
>>>>>>>> +"No Firmware Assisted Non-Maskable Interrupts support in TCG, try cap-fwnmi-mce=off");
>>>>>>>
>>>>>>> Not allowing this for TCG creates an awkward incompatibility between
>>>>>>> KVM and TCG guests.  I can't actually see any reason to ban it for TCG
>>>>>>> - with the current code TCG won't ever generate NMIs, but I don't see
>>>>>>> that anything will actually break.
>>>>>>>
>>>>>>> In fact, we do have an nmi monitor command, currently wired to the
>>>>>>> spapr_nmi() function which resets each cpu, but it probably makes
>>>>>>> sense to wire it up to the fwnmi stuff when present.
>>>>>>
>>>>>> Yes, but that nmi support is not enough to inject a synchronous error
>>>>>> into the guest kernel. For example, we should provide the faulty address
>>>>>> along with other information such as the type of error (slb multi-hit,
>>>>>> memory error, TLB multi-hit) and when the error occurred (load/store)
>>>>>> and whether the error was completely recovered or not. Without such
>>>>>> information we cannot build the error log and pass it on to the guest
>>>>>> kernel. Right now nmi monitor command takes cpu number as the only argument.
>>>>>
>>>>> Obviously we can't inject an arbitrary MCE event with that monitor
>>>>> command.  But isn't there some sort of catch-all / unknown type of MCE
>>>>> event which we could inject?
>>>>
>>>> We have "unknown" type of error, but we should also pass an address in
>>>> the MCE event log. Strictly speaking this address should be a valid
>>>> address in the current CPU context as MCEs are synchronous errors
>>>> triggered when we touch a bad address.
>>>
>>> Well, some of them are.  At least historically both synchronous and
>>> asnchronous MCEs were possible.  Are there really no versions where
>>> you can report an MCE with unknown address?
>>
>> I am not aware of any such versions. Will cross check.
>>
>>>
>>>> We can pass a default address with every nmi, but I am not sure whether
>>>> that will be practically helpful.
>>>>
>>>>> It seems very confusing to me to have 2 totally separate "nmi"
>>>>> mechanisms.
>>>>>
>>>>>> So I think TCG support should be a separate patch by itself.
>>>>>
>>>>> Even if we don't wire up the monitor command, I still don't see
>>>>> anything that this patch breaks - we can support the nmi-register and
>>>>> nmi-interlock calls without ever actually creating MCE events.
>>>>
>>>> If we support nmi-register and nmi-interlock calls without the monitor
>>>> command wire-up then we will be falsely claiming the nmi support to the
>>>> guest while it is not actually supported.
>>>
>>> How so?  AFAICT, from the point of view of the guest this is not
>>> observably different from supporting the NMI mechanism but NMIs never
>>> occurring.
>>
>> A guest inserting a duplicate SLB will expect the machine check
>> exception delivered to the handler registered via nmi,register.
>> But we actually don't do that in TCG.
> 
> Ah, true, I was thinking of external hardware fault triggered MCEs
> rather than software error ones like duplicate SLB.
> 
> That said, I strongly suspect TCG is buggy enough at present that
> exact behaviour in rare error conditions like duplicate SLB is not
> really a big problem in the scheme of things.
> 
> I really don't think we can enable this by default until we allow it
> for TCG - we don't want starting a TCG guest to involve manually
> switching other options.
> 
> We could consider allowing it for TCG but just printing a warning that
> the behaviour may not be correct in some conditions - we do something
> similar for some of the Spectre workarounds already.

I think we better not enable this by default until we enhance TCG to
support fwnmi.

> 

-- 
Regards,
Aravinda