Re: [PATCH v2 6/7] x68: acpi: trigger SMI before sending hotplug Notify event to OSPM

[Laszlo Ersek]

On 08/26/20 11:24, Laszlo Ersek wrote:
> Hi Igor,
> 
> On 08/25/20 19:25, Laszlo Ersek wrote:
> 
>> So I would suggest fetching the CNEW array element back into "uid"
>> first, then using "uid" for both the NOTIFY call, and the (currently
>> missing) restoration of CSEL. Then we can write 1 to CINS.
>>
>> Expressed as a patch on top of yours:
>>
>>> diff --git a/hw/acpi/cpu.c b/hw/acpi/cpu.c
>>> index 4864c3b39694..2bea6144fd5e 100644
>>> --- a/hw/acpi/cpu.c
>>> +++ b/hw/acpi/cpu.c
>>> @@ -564,8 +564,11 @@ void build_cpus_aml(Aml *table, MachineState *machine, 
>>> CPUHotplugFeatures opts,
>>>              aml_append(method, aml_store(zero, cpu_idx));
>>>              while_ctx = aml_while(aml_lless(cpu_idx, num_added_cpus));
>>>              {
>>> -                aml_append(while_ctx, aml_call2(CPU_NOTIFY_METHOD,
>>> -                    aml_derefof(aml_index(new_cpus, cpu_idx)), dev_chk));
>>> +                aml_append(while_ctx,
>>> +                    aml_store(aml_derefof(aml_index(new_cpus, cpu_idx)), 
>>> uid));
>>> +                aml_append(while_ctx,
>>> +                    aml_call2(CPU_NOTIFY_METHOD, uid, dev_chk));
>>> +                aml_append(while_ctx, aml_store(uid, cpu_selector));
>>>                  aml_append(while_ctx, aml_store(one, ins_evt));
>>>                  aml_append(while_ctx, aml_increment(cpu_idx));
>>>              }
>>
>> This effects the following change, in the decompiled method:
>>
>>> @@ -37,15 +37,17 @@
>>>      If ((Local_NumAddedCpus != Zero))
>>>      {
>>>          \_SB.PCI0.SMI0.SMIC = 0x04
>>>      }
>>>
>>>      Local_CpuIdx = Zero
>>>      While ((Local_CpuIdx < Local_NumAddedCpus))
>>>      {
>>> -        CTFY (DerefOf (CNEW [Local_CpuIdx]), One)
>>> +        Local_Uid = DerefOf (CNEW [Local_CpuIdx])
>>> +        CTFY (Local_Uid, One)
>>> +        \_SB.PCI0.PRES.CSEL = Local_Uid
>>>          \_SB.PCI0.PRES.CINS = One
>>>          Local_CpuIdx++
>>>      }
>>>
>>>      Release (\_SB.PCI0.PRES.CPLK)
>>>  }
>>
>> With this change, the
>>
>>   virsh setvcpus DOMAIN 8 --live
>>
>> command works for me. The topology in my test domain has CPU#0 and
>> CPU#2 cold-plugged, so the command adds 6 VCPUs. Viewed from the
>> firmware side, the 6 "device_add" commands, issued in close succession
>> by libvirtd, coalesce into 4 "batches". (And of course the firmware
>> sees the 4 batches back-to-back.)
> 
> unfortunately, with more testing, I have run into two more races:
> 
> (1) When a "device_add" occurs after the ACPI loop collects the CPUS
>     from the register block, but before the SMI.
> 
>     Here, the "stray CPU" is processed fine by the firmware. However,
>     the CTFY loop in ACPI does not know about the CPU, so it doesn't
>     clear the pending insert event for it. And when the firmware is
>     entered with an SMI for the *next* time, the firmware sees the same
>     CPU *again* as pending, and tries to relocate it again. Bad things
>     happen.
> 
> (2) When a "device_add" occurs after the SMI, but before the firmware
>     collects the pending CPUs from the register block.
> 
>     Here, the firmware collects the "stray CPU". However, the "broadcast
>     SMI", with which we entered the firmware, did *not* cover the stray
>     CPU -- the CPU_FOREACH() loop in ich9_apm_ctrl_changed() could not
>     make the SMI pending for the new CPU, because at that time, the CPU
>     had not been added yet. As a result, when the firmware sends an
>     INIT-SIPI-SIPI to the new CPU, expecting it to boot right into SMM,
>     the new CPU instead boots straight into the post-RSM (normal mode)
>     "pen", skipping its initial SMI handler. Meaning that the CPU halts
>     nicely, but its SMBASE is never relocated, and the SMRAM message
>     exchange with the BSP falls apart.
> 
> Possible mitigations I can think of:
> 
> For problem (1):
> 
>   (1a) Change the firmware so it notices that it has relocated the
>        "stray" CPU before -- such CPUs should be simply skipped in the
>        firmware. The next time the CTFY loop runs in ACPI, it will clear
>        the pending event.
> 
>   (1b) Alternatively, stop consuming the hotplug register block in the
>        firmware altogether, and work out general message passing, from
>        ACPI to firmware. See the outline here:
> 
>          
> http://mid.mail-archive.com/cf887d74-f65d-602a-9629-3d25cef93a69@redhat.com
> 
> For problem (2):
> 
>   (2a) Change the firmware so that it sends a directed SMI as well to
>        each CPU, just before sending an INIT-SIPI-SIPI. This should be
>        idempotent -- if the broadcast SMI *has* covered the the CPU,
>        then sending a directed SMI should make no difference.
> 
>   (2b) Alternatively, change the "device_add" command in QEMU so that,
>        if "CPU hotplug with SMI" has been negotiated, the new CPU is
>        added with the SMI made pending for it at once. (That is, no
>        hot-plugged CPU would exist with the directed SMI *not* pending
>        for it.)
> 
>   (2c) Alternatively, approach (1b) would fix problem (2) as well -- the
>        firmware would only relocate such CPUs that ACPI collected before
>        injecting the SMI. So all those CPUs would have the SMI pending.
> 
> 
> I can experiment with (1a) and (2a),

My patches for (1a) and (1b) seem to work -- my workstation has 10
PCPUs, and I'm using a guest with 20 possible VCPUs and 2 cold-plugged
VCPUs on it, for testing. The patches survive the hot-plugging of 18
VCPUs in one go, or two batches like 9+9. I can see the fixes being
exercised.

Unless you strongly disagree (or I find issues in further testing), I
propose that I post these fixes to edk2-devel (they should still be in
scope for the upcoming release), and that we stick with your current
patch series for QEMU (v3 -- upcoming, or maybe already posted).

Thanks!
Laszlo