Re: [Xen-devel] [for-4.7] x86/emulate: synchronize LOCKed instruction emulation

["Jan Beulich" <jbeulich@xxxxxxxx>]

>>> Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx> 04/19/16 1:01 PM >>>
>I think this might be because the LOCK prefix should guarantee that the
>instruction that follows it has exclusive use of shared memory (for both
>reads and writes) but I might be misreading the docs:

LOCK definitely has no effect on other than the instruction it gets applied
to.

Jan

>(From the Intel SDM) "Causes the processor’s LOCK# signal to be asserted
>during execution of the accompanying instruction (turns the instruction
>into an atomic instruction). In a multiprocessor environment, the LOCK#
>signal ensures that the processor has exclusive use of any shared memory
>while the signal is asserted."
>
>Using a spin lock (or the rwlock in the manner described above) in
>hvmemul_cmpxchg() only prevents two LOCKed instructions from running at
>the same time, but presumably even non-LOCKed instructions just reading
>that memory area should be prevented from running until the LOCKed
>instruction is done (hence the guest starting up with the rwlock in
>x86_emulate() but not with it locked only in hvmemul_cmpxchg()).
>
>Hopefully I haven't misunderstood the issue.
>
>
>Thanks,
>Razvan



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