Re: [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split lock

[Sean Christopherson]

On Wed, Oct 16, 2019 at 05:43:53PM +0200, Paolo Bonzini wrote:
> On 16/10/19 17:41, Sean Christopherson wrote:
> > On Wed, Oct 16, 2019 at 04:08:14PM +0200, Paolo Bonzini wrote:
> >> SIGBUS (actually a new KVM_EXIT_INTERNAL_ERROR result from KVM_RUN is
> >> better, but that's the idea) is for when you're debugging guests.
> >> Global disable (or alternatively, disable SMT) is for production use.
> > 
> > Alternatively, for guests without split-lock #AC enabled, what if KVM were
> > to emulate the faulting instruction with split-lock detection temporarily
> > disabled?
> 
> Yes we can get fancy, but remember that KVM is not yet supporting
> emulation of locked instructions.  Adding it is possible but shouldn't
> be in the critical path for the whole feature.

Ah, didn't realize that.  I'm surprised emulating all locks with cmpxchg
doesn't cause problems (or am I misreading the code?).  Assuming I'm
reading the code correctly, the #AC path could kick all other vCPUS on
emulation failure and then retry emulation to "guarantee" success.  Though
that's starting to build quite the house of cards.

> How would you disable split-lock detection temporarily?  Just tweak
> MSR_TEST_CTRL for the time of running the one instruction, and cross
> fingers that the sibling doesn't notice?

Tweak MSR_TEST_CTRL, with logic to handle the scenario where split-lock
detection is globally disable during emulation (so KVM doesn't
inadvertantly re-enable it).

There isn't much for the sibling to notice.  The kernel would temporarily
allow split-locks on the sibling, but that's a performance issue and isn't
directly fatal.  A missed #AC in the host kernel would only delay the
inevitable global disabling of split-lock.  A missed #AC in userspace would
again just delay the inevitable SIGBUS.