Re: [Xen-devel] [PATCH RFC] x86/emulate: implement hvmemul_cmpxchg() with an actual CMPXCHG

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 28.03.17 at 11:14, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> I'm not sure that the RETRY model is what the guest OS expects. AFAIK, a
> failed CMPXCHG should happen just once, with the proper registers and ZF
> set. The guest surely expects neither that the instruction resume until
> it succeeds, nor that some hidden loop goes on for an undeterminate
> ammount of time until a CMPXCHG succeeds.

The guest doesn't observe the CMPXCHG failing - RETRY leads to
the instruction being restarted instead of completed.

There is a severe downside to MMIO accesses with this model
though: Other than for RAM, we shouldn't be reading (and even
less so writing) the memory location more than once.

> The picture is further complicated by the two-part handling of
> instructions in x86_emulate(). For example, for CMPXCHG we have:
>[...]
> I now see that this is why using a spinlock only around the writeback
> part did not solve the issue: both the compare part and the writeback
> part need to be part of the same atomic operation - lock needs to be
> aquired before the cmp / ZF part.

No exactly: RMW insns require the lock to be acquired ahead of
the memory read, and dropped after the memory write.

> Opinions and suggestions are appreciated. If I'm not mistaken, it looks
> like the smp_lock design is the better solution to the problem.

Andrew has told me that the re-work of how we do memory accesses
in the emulator is pretty high on his priority list now. Whether we'd
want to introduce an interim solution therefore depends on the time
scale to possibly reach the only ultimately correct solution (no longer
acting on intermediate copies of the data coming from guest memory,
which is only correct for plain reads and plain writes).

Jan


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