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

[Laszlo Ersek]

On 08/26/20 15:32, Laszlo Ersek wrote:
> 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

aargh, I meant my patches for (1a) and (2a).

sorry
Laszlo

> 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
>