Re: [patch 2/5] KVM: reintroduce guest mode bit and unify remote request code

[Marcelo Tosatti <mtosatti@redhat.com>]

On Thu, Aug 27, 2009 at 04:24:58PM +0300, Avi Kivity wrote:
> On 08/27/2009 03:45 PM, Marcelo Tosatti wrote:
>>> Why not bool guest_mode?  Saves two atomics per exit.
>>>      
>> It must be atomic since GUEST_MODE / VCPU_KICKED bits are manipulated by
>> multiple CPU's.
>>    
>
> bools are atomic.

OK.

- VCPU_KICKED requires test_and_set. GUEST_MODE/VCPU_KICKED accesses
  must not be reordered.

(OK, could have GUEST_MODE in a bool even so, but its easier to read
by keeping them together, at least to me).

- Its easier to cacheline align with longs rather than bools?

- From testing it seems the LOCK prefix is not heavy, as long as its
  cpu local (probably due to 7.1.4 Effects of a LOCK Operation on
  Internal Processor Caches?).

BTW, 

7.1.2.2 Software Controlled Bus Locking

Software should access semaphores (shared memory used for signalling
between multiple processors) using identical addresses and operand
lengths. For example, if one processor accesses a semaphore using a word
access, other processors should not access the semaphore using a byte
access.

The bit operations use 32-bit access, but the vcpu->requests check in
vcpu_enter_guest uses 64-bit access.

Is that safe?