Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII;
 format=flowed
Content-Transfer-Encoding: 7bit
Date:   Wed, 02 Dec 2020 08:18:03 +0000
From:   Marc Zyngier <maz@kernel.org>
To:     Will Deacon <will@kernel.org>
Cc:     Quentin Perret <qperret@google.com>,
        linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Qais Yousef <qais.yousef@arm.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Tejun Heo <tj@kernel.org>, Li Zefan <lizefan@huawei.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        kernel-team@android.com
Subject: Re: [PATCH v4 03/14] KVM: arm64: Kill 32-bit vCPUs on systems with
 mismatched EL0 support
In-Reply-To: <20201201165707.GF27783@willie-the-truck>
References: <20201124155039.13804-1-will@kernel.org>
 <20201124155039.13804-4-will@kernel.org>
 <9bd06b193e7fb859a1207bb1302b7597@kernel.org>
 <20201127115304.GB20564@willie-the-truck>
 <583c4074bbd4cf8b8085037745a5d1c0@kernel.org>
 <20201127172434.GA984327@google.com>
 <9de8639549040b4478b312503fd5a23f@kernel.org>
 <20201201165707.GF27783@willie-the-truck>
User-Agent: Roundcube Webmail/1.4.9
Message-ID: <5e59a8f5bc84403ce2c8f26aa874cb1b@kernel.org>
Precedence: bulk

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...