Re: [PATCH v4 03/14] KVM: arm64: Kill 32-bit vCPUs on systems with mismatched EL0 support

[Marc Zyngier]

On 2020-12-01 16:57, Will Deacon wrote:
> On Fri, Nov 27, 2020 at 06:16:35PM +0000, Marc Zyngier wrote:
> > On 2020-11-27 17:24, Quentin Perret wrote:
> > > On Friday 27 Nov 2020 at 17:14:11 (+0000), Marc Zyngier wrote:
> >
> > [...]
> >
> > > > Yeah, the sanitized read feels better, if only because that is
> > > > what we are going to read in all the valid cases, unfortunately.
> > > > read_sanitised_ftr_reg() is sadly not designed to be called on
> > > > a fast path, meaning that 32bit guests will do a bsearch() on
> > > > the ID-regs every time they exit...
> > > >
> > > > I guess we will have to evaluate how much we loose with this.
> > >
> > > Could we use the trick we have for arm64_ftr_reg_ctrel0 to speed this
> > > up?
> >
> > Maybe. I want to first verify whether this has any measurable impact.
> > Another possibility would be to cache the last 
> > read_sanitised_ftr_reg()
> > access, just to see if that helps. There shouldn't be that many code
> > paths hammering it.
>
> We don't have huge numbers of ID registers, so the bsearch shouldn't be
> too expensive. However, I'd like to remind myself why we can't index 
> into
> the feature register array directly as we _should_ know all of this 
> stuff
> at compile time, right?

Simply because it's not indexed by ID reg. It's just an ordered 
collection,
similar to the for sys_reg emulation in KVM. You can compute the index
ahead of time, but just not at compile time. At least not with the
way the arm64_ftr_regs array is built.

        M.
--
Jazz is not dead. It just smells funny...