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

["Michael S. Tsirkin" <mst@redhat.com>]

On Sun, Apr 12, 2015 at 06:14:10PM +0000, James Bottomley wrote:
> On Sun, 2015-04-12 at 19:20 +0200, Michael S. Tsirkin wrote:
> > On Sun, Apr 12, 2015 at 04:34:05PM +0000, James Bottomley wrote:
> > > 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
> > > 
> > 
> > It's non normative because it doesn't apply to any existing devices.
> > It's a guideline for future devices.
> 
> But it does apply to current devices: no current device has an action
> field wider than 32 bits, so they all currently obey it.
> 
> > How about:
> > 
> > Therefore, it is preferable not to make writeable fields wider than 32
> > bits. In any case, do not make writes into fields wider than 32 
> > bits trigger device actions.
> 
> It's a bit verbose.  Clarity is usually in simplicity.  The simplest way
> to express should in English without actually saying "should" is "ought
> to", so
> 
>         Therefore, no writeable field which triggers an action ought to
>         be wider than 32 bits
> 
> James


OK, that works.