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:   Fri, 27 Nov 2020 18:16:35 +0000
From:   Marc Zyngier <maz@kernel.org>
To:     Quentin Perret <qperret@google.com>
Cc:     Will Deacon <will@kernel.org>,
        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: <20201127172434.GA984327@google.com>
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>
User-Agent: Roundcube Webmail/1.4.9
Message-ID: <9de8639549040b4478b312503fd5a23f@kernel.org>
Precedence: bulk

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.

Thanks,

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