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Subject: Re: [PATCH 9/9] arm64: Retrieve stolen time as paravirtualized guest
To:     Steven Price <steven.price@arm.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>,
        Russell King <linux@armlinux.org.uk>,
        Will Deacon <will@kernel.org>,
        James Morse <james.morse@arm.com>,
        Julien Thierry <julien.thierry.kdev@gmail.com>,
        Suzuki K Pouloze <suzuki.poulose@arm.com>,
        kvm@vger.kernel.org, kvmarm@lists.cs.columbia.edu,
        linux-arm-kernel@lists.infradead.org, linux-doc@vger.kernel.org,
        linux-kernel@vger.kernel.org
References: <20190802145017.42543-1-steven.price@arm.com>
 <20190802145017.42543-10-steven.price@arm.com> <20190804105353.5e9824dc@why>
 <dc8a1e56-7b52-cc8f-265d-27eb5f458613@arm.com>
From:   Marc Zyngier <maz@kernel.org>
Organization: Approximate
Message-ID: <7108a70b-dafd-507b-8509-f4a092ef24af@kernel.org>
Date:   Thu, 8 Aug 2019 16:49:45 +0100
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On 08/08/2019 16:29, Steven Price wrote:
> On 04/08/2019 10:53, Marc Zyngier wrote:
>> On Fri,  2 Aug 2019 15:50:17 +0100
>> Steven Price <steven.price@arm.com> wrote:
>>
>>> Enable paravirtualization features when running under a hypervisor
>>> supporting the PV_TIME_ST hypercall.
>>>
>>> For each (v)CPU, we ask the hypervisor for the location of a shared
>>> page which the hypervisor will use to report stolen time to us. We set
>>> pv_time_ops to the stolen time function which simply reads the stolen
>>> value from the shared page for a VCPU. We guarantee single-copy
>>> atomicity using READ_ONCE which means we can also read the stolen
>>> time for another VCPU than the currently running one while it is
>>> potentially being updated by the hypervisor.
>>>
>>> Signed-off-by: Steven Price <steven.price@arm.com>
>>> ---
>>>  arch/arm64/kernel/Makefile |   1 +
>>>  arch/arm64/kernel/kvm.c    | 155 +++++++++++++++++++++++++++++++++++++

[...]

>>> +static int __init kvm_guest_init(void)
>>> +{
>>> +	int ret = 0;
>>> +
>>> +	if (!has_kvm_steal_clock())
>>> +		return 0;
>>> +
>>> +	ret = kvm_arm_init_stolen_time();
>>> +	if (ret)
>>> +		return ret;
>>> +
>>> +	pv_ops.time.steal_clock = kvm_steal_clock;
>>> +
>>> +	static_key_slow_inc(&paravirt_steal_enabled);
>>> +	if (steal_acc)
>>> +		static_key_slow_inc(&paravirt_steal_rq_enabled);
>>> +
>>> +	pr_info("using stolen time PV\n");
>>> +
>>> +	return 0;
>>> +}
>>> +early_initcall(kvm_guest_init);
>>
>> Is there any reason why we wouldn't directly call into this rather than
>> using an initcall?
> 
> I'm not sure where the direct call would go - any pointers?

I'd be temped to say arch/arm64/kernel/time.c:time_init(), provided that
there is no issue with the CPU hotplug lock (I remember hitting that a
while ago).

	M.
-- 
Jazz is not dead, it just smells funny...