virtio — archive

[PATCH] give explicit guidance on the use of 64 bit fields

From: Michael S. Tsirkin <[email protected]>
To: [email protected]
Date: Sun, 12 Apr 2015 19:13:00 -0000

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 <[email protected]> > > > > > > > > > > --- > > > > > 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.