[Bug 1735049] Re: Need MTTCG support for x86 guests

[Emilio G. Cota]

Looks like support for this was not fully added; my apologies for
closing this bug too early.

Adding full support for strong-on-weak emulation would be simple, at
least when it comes to memory ordering. The slowdown would be huge
though, see Figure 12 in
http://www.cs.columbia.edu/~cota/pubs/cota_cgo17.pdf (i.e. ~2x hmean
overhead for SPEC).

The good news is that with hardware support this overhead is ~0 (see SAO
in that figure).

The other feature that is not yet implemented in upstream QEMU is the
correct emulation of LL/SC, although for most code out there this
shouldn't be an issue in practice given that most parallel code relies
on cmpxchg, not on LL/SC pairs.

I'm reopening this bug an Cc'ing a few people who are more familiar with
the current code than I am in case I missed anything.

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https://bugs.launchpad.net/bugs/1735049

Title:
  Need MTTCG support for x86 guests

Status in QEMU:
  Fix Released

Bug description:
  MTTCG support is notably absent for x86_64 guests.  The last Wiki
  update on MTTCG was back in 2015, and I am having some difficulty
  determining the current status of the underlying requirements to
  enable this feature on x86 hosts.

  For instance, has support for strong-on-weak memory consistency been
  added into QEMU GIT at this point?

  Thanks!

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