Re: [Qemu-devel] when does a target frontend need to use gen_io_start()/gen_io_end() ?

[Paolo Bonzini <pbonzini@redhat.com>]

On 12/05/2015 21:41, Peter Maydell wrote:
>> > It's any instruction that can cause an icount read, typically through
>> > QEMU_CLOCK_VIRTUAL or cpu_get_ticks().
> Also anything that can cause a CPU interrupt, since tcg_handle_interrupt()
> will call cpu_abort() if the CPU gets an interrupt while it's not
> in a 'can do IO' state.
> 
> Anything else?
> 
> [How are -icount and multi-threaded TCG going to interact? Do we
> just say "you get one or the other but not both" ?]

For -icount and SMP, yes.  I even posted a patch to that end once.

You can get -icount and multi-threaded TCG (which for UP is simply TCG
with execution out of the BQL) together I think.  For example you could
handle cpu->icount_decr.u16.low == 0 like cpu->halted, hanging the CPU
thread until QEMU_CLOCK_VIRTUAL timers have been processed.  The I/O
thread would have to kick the CPU after processing QEMU_CLOCK_VIRTUAL
timers---not hard to do.

In fact, I suspect cpu->halted should become a kind of bitmap, and "wait
for interrupt" should be just one bit in there.  Any operation that
requires synchronization with other VCPUs should use cpu->halted so that
VCPUs can still run foreign code with run_on_vcpu.  This was the plan I
outlined to Frederic and Mark for flushing TLB remotely, at least.

Paolo