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. Now the question is whether we have managed to *sufficiently* imitate these patterns from PiSmmCpuDxeSmm, in this patch set. Making stuff volatile, and relying on the existent sync-up point, might suffice. Thanks Laszlo -=-=-=-=-=-=-=-=-=-=-=- Groups.io Links: You receive all messages sent to this group. View/Reply Online (#71067): https://edk2.groups.io/g/devel/message/71067 Mute This Topic: https://groups.io/mt/80199926/21656 Group Owner: devel+ow...@edk2.groups.io Unsubscribe: https://edk2.groups.io/g/devel/unsub [arch...@mail-archive.com] -=-=-=-=-=-=-=-=-=-=-=-
