Re: [virtio-dev] [PATCH] give explicit guidance on the use of 64 bit fields

[James Bottomley <jbottomley@odin.com>]

On Sun, 2015-04-12 at 18:22 +0200, Michael S. Tsirkin wrote:
> On Sun, Apr 12, 2015 at 04:06:28PM +0000, James Bottomley wrote:
> > Just saying 64 bit fields may not be atomic is true, but less helpful
> > than it might be.  Add explicit guidance about what the consequences of
> > non-atomicity are
> > 
> > Signed-off-by: James Bottomley <JBottomley@Odin.com>
> > 
> > ---
> > diff --git a/newdevice.tex b/newdevice.tex
> > index c7e6221..12123b9 100644
> > --- a/newdevice.tex
> > +++ b/newdevice.tex
> > @@ -24,6 +24,8 @@ large).
> >  
> >  Remember that configuration fields over 32 bits wide might not be
> >  atomically writable by the driver.
> > +Therefore, no writeable field which triggers an action should be wider than 32
> > +bits.
> >  
> >  \section{What Device Number?}\label{sec:Creating New Device Types / What Device Number?}
> >  
> > 
> > 
> 
> 
> It's best to avoid the word "should" outside normative statements.
> Can you rephrase this?

I've got to say that's a bit daft: should is the standard verbal
modifier for conditional or subjunctive ... it's quite a big chunk of
the english language.

However, to be honest, I think the statement needs to be normative.
It's not advisory: if someone creates a 64 bit action field the spec
becomes invalid on 32 bit architectures.

The only valid reason to ignore the statement is if your action field
has a specifically defined trigger (like only 32 bit writes to the lower
field triggers the action, so you write upper first then lower).  That's
why I think it's "should" rather than "may not".

James


James