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

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Date:   Wed, 02 Dec 2020 08:18:03 +0000
From:   Marc Zyngier <maz@...nel.org>
To:     Will Deacon <will@...nel.org>
Cc:     Quentin Perret <qperret@...gle.com>,
        linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Qais Yousef <qais.yousef@....com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Tejun Heo <tj@...nel.org>, Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        kernel-team@...roid.com
Subject: Re: [PATCH v4 03/14] KVM: arm64: Kill 32-bit vCPUs on systems with
 mismatched EL0 support

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...