On Wed, 03 Aug 2016, Michael S. Tsirkin wrote:
> > And I'm still discussing this with the hardware people. It seems we
> > can do this for *most* things, but not all; the question is where
> > exactly we need to do something different.
Let's hope the "hardware guys" get back to you soon :(
HSD162/BDM116 MOVNTDQA From WC Memory May Pass Earlier Locked
Instructions
Problem: An execution of (V)MOVNTDQA (streaming load instruction)
that loads from WC (write combining) memory may appear to pass an
earlier locked instruction that accesses a different cache line.
Implication: Software that expects a lock to fence subsequent
(V)MOVNTDQA instructions may not operate properly.
Workaround: None identified. Software that relies on a locked
instruction to fence subsequent executions of (V)MOVNTDQA should
insert an MFENCE instruction between the locked instruction and
subsequent (V)MOVNTDQA instruction.
SKL079 MOVNTDQA From WC Memory May Pass Earlier MFENCE Instructions
Problem: An execution of MOVNTDQA or VMOVNTDQA that loads from WC
(write combining) memory may appear to pass an earlier execution of
the MFENCE instruction.
Implication: When this erratum occurs, an execution of MOVNTDQA or
VMOVNTDQA may appear to execute before memory operations that
precede the earlier MFENCE instruction. Software that uses MFENCE
to order subsequent executions of the MOVNTDQA instructions may not
operate properly.
Workaround: It is possible for the BIOS to contain a workaround for
this erratum. For the steppings affected, see the Summary Table of
Changes.
These are just examples. Intel might have other errata related to
*FENCE or LOCK, and AMD might have its share of model-specific LOCK or
*FENCE oddities as well (I didn't check).
Note that Skylake is broken in exactly the opposite way that Haswell and
Broadwell are. Fortunately, Skylake could be fixed through a microcode
update, but still...
The point is that we indeed need to be careful if we want to switch away
from *FENCE.
--
"One disk to rule them all, One disk to find them. One disk to bring
them all and in the darkness grind them. In the Land of Redmond
where the shadows lie." -- The Silicon Valley Tarot
Henrique Holschuh
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