Re: [PATCH 4/8] edu: add smp_mb__after_rmw()

[Paolo Bonzini]

On 3/6/23 14:38, Peter Maydell wrote:
> On Fri, 3 Mar 2023 at 17:21, Paolo Bonzini <pbonzini@redhat.com> wrote:
> > Ensure ordering between clearing the COMPUTING flag and checking
> > IRQFACT, and between setting the IRQFACT flag and checking
> > COMPUTING.  This ensures that no wakeups are lost.
> >
> > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>
> Why is this device even messing around with multiple
> threads and atomics anyway ??

Because it is an example of deferring device work to another thread, 
just like on real hardware it may be deferred to an on-device 
microcontroller or CPU.

In particular, in this case I think it is useful to show a pitfall that 
is present in emulated hardware (where all loads and stores ultimately 
go through a store buffer and CPU cache) and not in real hardware (where 
I/O registers are always uncached).

> I'm quite tempted to suggest we should deprecate-and-drop
> this; it's not real hardware, it doesn't do anything
> useful, and it's not a good model to follow if you're
> implementing some other device.

I'm okay with deprecating it but I think it has educational value.

Paolo