An interesting use case is to defeat speculation attacks. See
https://github.com/torvalds/linux/blob/master/arch/x86/include/asm/nospec-branch.h.
On 09/10/2019 05.12, Vitaly Davidovich wrote:
FWIW, I’ve only seen lfence used precisely in the 2 cases mentioned in
this thread:
1) use of non-temporal loads (ie weak ordering, normal x86 guarantees
go out the window)
2) controlling execution of non-serializing instructions like rdtsc
I’d be curious myself to hear of other cases.
On Fri, Oct 4, 2019 at 10:10 AM Peter Veentjer <[email protected]
<mailto:[email protected]>> wrote:
I'm have been checking out the new fence API's in Java
(Unsafe/VarHandle).
I understand how the higher level API are translated to the
logical fences. E.g. release fence -> LoadStore+StoreStore. There
are some great post including
https://shipilev.net/blog/2014/on-the-fence-with-dependencies/
Great explanation how a release fence needs to be combined with a
StoreLoad to preserve sequential consistency
Also this post is great on the topic:
https://preshing.com/20120913/acquire-and-release-semantics/
When I zoom into hardware things are a bit more blurry.
X86 provides the following guarantees:
Loads won't be reordered with older loads [LoadLoad]
Stores won't be reordered with older stores (TSO) [StoreStore]
Stores won't be reordered with older loads [LoadStore]
One fundamental fence is the MFENCE because it will provide
StoreLoad semantics. And on X86 the Unsafe.fullFence can be
compiled to a MFENCE (in practice it uses the lock addl ... but
that isn't relevant for this discussion). This will prevent stores
to be reordered with older stores and will make sure the memory is
visible to other CPU's (by waiting for the store buffer to be
drained).
I think you meant “prevent stores to be reordered with *later
loads*”. In fact, awaiting store buffer drain is how it prevents the
later load from reordering with an earlier store - the load can’t
retire (maybe not even issue) while the store is sitting in the buffer
(which would cause the load-before-store reordering to be observed).
The SFENCE was a bit more obscure to be because X86 proves TSO; so
what is the point of adding a [StoreStore] fence is the platform
provides it out of the box (so prevents stores to be reordered
with older stores). Apparently there are certain instructions like
those of SSE that are weakly ordered and these need to have this
SFENCE. Ok. I can live with that.
But the LFENCE I can't place. Initially I thought it would provide
a similar fix as the SFENCE; so prevent load load reordering for
weakly ordered instructions like those of SSE. But apparently the
LFENCE is a very different beast.
Could someone shed some light on the purpose of the LFENCE?
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