Re: [PATCH 3/3] xen/arm: Add sb instruction support

[Julien Grall <julien@xxxxxxx>]

On 09/05/2022 11:08, Bertrand Marquis wrote:
> Hi Julien,
Hi,

> > On 4 May 2022, at 09:06, Julien Grall <julien@xxxxxxx> wrote:
> >
> >
> >
> > On 04/05/2022 08:24, Bertrand Marquis wrote:
> > > Hi Julien,
> > Hi Bertrand,
> >
> > > > On 3 May 2022, at 19:47, Julien Grall <julien@xxxxxxx> wrote:
> > > > > A new cpuerrata capability is introduced to enable the alternative
> > > > 'sb' is definitely not an erratum. Errata are for stuff that are meant to be 
> > > > specific to one (or multiple) CPU and they are not part of the architecture.
> > > >
> > > > This is the first time we introduce a feature in Xen. So we need to add a new 
> > > > array in cpufeature.c that will cover 'SB' for now. In future we could add 
> > > > feature like pointer auth, LSE atomics...
> > > I am not quite sure why you would want to do that.
> > > Using the sb instruction is definitely something to do to solve erratas, if a 
> > > CPU is not impacted by those erratas, why would you want to use this ?
> > I agree that SB is used to solve errata but the instruction itself is not a 
> > workaround (it may be part of it though). Instead, this is a more efficient way 
> > to prevent speculation and will replace dsb/isb.
> >
> > Speculation is never going to disappear from processor. So, in the future, there might be 
> > valid reason for us to say "We don't want the processor to speculate". This 
> > would mean using SB.
> If the need arise then we will check depending on that how we can support it but in 
> the current status as there is no use case I don’t think we need that.
It is not clear how I should read this answer... If you add SB in 
cpuerrata.c, then a user will start to see message like:
"enabled workaround for Speculation Barrier".

Which is completely bogus. Replacing "dsb; isb" with "sb" is mostly an 
optimization and none of the current use will end up to be 
architecturaly executed.
I appreciate this is more work to add cpufeature.c. However, AFAIK, 
there are no rush to get this optimization in (see why above) and muddy 
(even temporarily) the logs.
Cheers,

--
Julien Grall