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

["Laszlo Ersek" <lersek@redhat.com>]

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

Thanks!
Laszlo