IOREQ completions for MMIO writes

[Jason Andryuk]

Hi Everyone,

I've been looking at ioreq latency and pausing of vCPUs.  Specifically 
for MMIO (IOREQ_TYPE_COPY) writes, they still need completions:

static inline bool ioreq_needs_completion(const ioreq_t *ioreq)
{
    return ioreq->state == STATE_IOREQ_READY &&
           !ioreq->data_is_ptr &&
           (ioreq->type != IOREQ_TYPE_PIO || ioreq->dir != IOREQ_WRITE);
}

state == STATE_IOREQ_READY
data_is_ptr == 0
type == IOREQ_TYPE_COPY
dir == IOREQ_WRITE

To a completion is needed.  The vCPU remains paused with 
_VPF_blocked_in_xen set in paused_flags until the ioreq server notifies 
of the completion.

At least for the case I'm looking, a single write to a mmio register, it 
doesn't seem like the vCPU needs to be blocked.  The write has been sent 
and subsequent emulation should not depend on it.

I feel like I am missing something, but I can't think of a specific 
example where a write needs to be blocking.  Maybe it simplifies the 
implementation, so a subsequent instruction will always have a ioreq 
slot available?

Any insights are appreciated.

Thanks,
Jason