Re: [edk2-devel] [PATCH v6 7/9] OvmfPkg/CpuHotplugSmm: add CpuEject()

["Ankur Arora" <ankur.a.arora@oracle.com>]

On 2021-02-05 8:06 a.m., Laszlo Ersek wrote:
> Hi Ankur,
> 
> I figure it's prudent for me to follow up here too:
> 
> On 02/04/21 03:49, Ankur Arora wrote:
>> On 2021-02-03 12:58 p.m., Laszlo Ersek wrote:
>>> On 02/03/21 07:45, Ankur Arora wrote:
>>>> On 2021-02-02 6:15 a.m., Laszlo Ersek wrote:
>>>>> On 02/02/21 15:00, Laszlo Ersek wrote:
>>>>>
>>>>>> ... I guess that volatile-qualifying both CPU_HOT_EJECT_DATA, and the
>>>>>> array pointed-to by CPU_HOT_EJECT_DATA.ApicIdMap, should suffice. In
>>>>>> combination with the sync-up point that you quoted. This seems to
>>>>>> match
>>>>>> existing practice in PiSmmCpuDxeSmm -- there are no concurrent
>>>>>> accesses,
>>>>>> so atomicity is not a concern, and serializing the instruction streams
>>>>>> coarsely, with the sync-up, in combination with volatile accesses,
>>>>>> should presumably guarantee visibility (on x86 anyway).
>>>>>
>>>>> To summarize, this is what I would ask for:
>>>>>
>>>>> - make CPU_HOT_EJECT_DATA volatile
>>>>>
>>>>> - make (*CPU_HOT_EJECT_DATA.ApicIdMap) volatile
>>>>>
>>>>> - after storing something to CPU_HOT_EJECT_DATA or
>>>>> CPU_HOT_EJECT_DATA.ApicIdMap on the BSP, execute a MemoryFence()
>>>>>
>>>>> - before fetching something from CPU_HOT_EJECT_DATA or
>>>>> CPU_HOT_EJECT_DATA.ApicIdMap on an AP, execute a MemoryFence()
>>>>>
>>>>>
>>>>> Except: MemoryFence() isn't a *memory fence* in fact.
>>>>>
>>>>> See "MdePkg/Library/BaseLib/X64/GccInline.c".
>>>>>
>>>>> It's just a compiler barrier, which may not add anything beyond what
>>>>> we'd already have from "volatile".
>>>>>
>>>>> Case in point: PiSmmCpuDxeSmm performs heavy multi-processing, but does
>>>>> not contain a single invocation of MemoryFence(). It uses volatile
>>>>> objects, and a handful of InterlockedCompareExchangeXx() calls, for
>>>>> implementing semaphores. (NB: there is no 8-bit variant of
>>>>> InterlockedCompareExchange(), as "volatile UINT8" is considered atomic
>>>>> in itself, and a suitable basis for a sempahore too.) And given the
>>>>> synchronization from those semaphores, PiSmmCpuDpxeSmm trusts that
>>>>> updates to the *other* volatile objects are both atomic and visible.
>>>>>
>>>>> I'm pretty sure this only works because x86 is in-order. There are
>>>>> instruction stream barriers in place, and compiler barriers too, but no
>>>>> actual memory barriers.
>>>>
>>>> Right and just to separate them explicitly, there are two problems:
>>>>
>>>>    - compiler: where the compiler caches stuff in or looks at stale
>>>> memory
>>>> locations. Now, AFAICS, this should not happen because the ApicIdMap
>>>> would
>>>> never change once set so the compiler should reasonably be able to cache
>>>> the address of ApicIdMap and dereference it (thus obviating the need for
>>>> volatile.)
>>>
>>> (CPU_HOT_EJECT_DATA.Handler does change though.)
>>
>> Yeah, I did kinda elide over that. Let me think this through in v7
>> and add more explicit comments and then we can see if it still looks
>> fishy?
>>
>> Thanks
>> Ankur
>>
>>>
>>>> The compiler could, however, cache any assignments to ApicIdMap[Idx]
>>>> (especially given LTO) and we could use a MemoryFence() (as the compiler
>>>> barrier that it is) to force the store.
>>>>
>>>>    - CPU pipeline: as you said, right now we basically depend on x86
>>>> store
>>>> order semantics (and the CpuPause() loop around AllCpusInSync, kinda
>>>> provides
>>>> a barrier.)
>>>>
>>>> So the BSP writes in this order:
>>>> ApicIdMap[Idx]=x; ... ->AllCpusInSync = true
>>>>
>>>> And whenever the AP sees ->AllCpusInSync == True, it has to now see
>>>> ApicIdMap[Idx] == x.
>>>
>>> Yes.
>>>
>>>>
>>>> Maybe the thing to do is to detail this barrier in a commit
>>>> note/comment?
>>>
>>> That would be nice.
>>>
>>>> And add the MemoryFence() but not the volatile.
>>>
>>> Yes, that should work.
> 
> Please *do* add the volatile, and also the MemoryFence(). When built
> with Visual Studio, MemoryFence() does nothing at all (at least when LTO
> is in effect -- which it almost always is). So we should have the
> volatile for making things work, and MemoryFence() as a conceptual
> reminder, so we know where to fix up things, when (if!) we come around
> fixing this mess with MemoryFence(). Reference:
> 
> https://edk2.groups.io/g/rfc/message/500
> https://edk2.groups.io/g/rfc/message/501
> https://edk2.groups.io/g/rfc/message/502
> https://edk2.groups.io/g/rfc/message/503

Did see it on the thread. Yeah agreed, Visual Studio does necessitate volatile here.
Will add.

Thanks
Ankur

> 
> Thanks!
> Laszlo
>