linux-kernel - Re: [PATCH v3 05/10] KVM: arm64: Support stolen time reporting via shared structure

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Message-ID: <301610cc-ba04-89d6-a0b6-d37ecf4a717a@arm.com>
Date:   Thu, 22 Aug 2019 12:00:54 +0100
From:   Steven Price <steven.price@....com>
To:     Jonathan Cameron <jonathan.cameron@...wei.com>
Cc:     Mark Rutland <mark.rutland@....com>, kvm@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        Suzuki K Pouloze <suzuki.poulose@....com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        Russell King <linux@...linux.org.uk>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Will Deacon <will@...nel.org>, kvmarm@...ts.cs.columbia.edu,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v3 05/10] KVM: arm64: Support stolen time reporting via
 shared structure

On 22/08/2019 11:39, Jonathan Cameron wrote:
> On Wed, 21 Aug 2019 16:36:51 +0100
> Steven Price <steven.price@....com> wrote:
> 
>> Implement the service call for configuring a shared structure between a
>> VCPU and the hypervisor in which the hypervisor can write the time
>> stolen from the VCPU's execution time by other tasks on the host.
>>
>> The hypervisor allocates memory which is placed at an IPA chosen by user
>> space. The hypervisor then updates the shared structure using
>> kvm_put_guest() to ensure single copy atomicity of the 64-bit value
>> reporting the stolen time in nanoseconds.
>>
>> Whenever stolen time is enabled by the guest, the stolen time counter is
>> reset.
>>
>> The stolen time itself is retrieved from the sched_info structure
>> maintained by the Linux scheduler code. We enable SCHEDSTATS when
>> selecting KVM Kconfig to ensure this value is meaningful.
>>
>> Signed-off-by: Steven Price <steven.price@....com>
> 
> One totally trivial comment inline... Feel free to ignore :)
> 
[...]
>> +int kvm_hypercall_stolen_time(struct kvm_vcpu *vcpu)
>> +{
>> +	u64 ret;
>> +	int err;
>> +
>> +	/*
>> +	 * Start counting stolen time from the time the guest requests
>> +	 * the feature enabled.
>> +	 */
>> +	vcpu->arch.steal.steal = 0;
>> +	vcpu->arch.steal.last_steal = current->sched_info.run_delay;
>> +
>> +	err = kvm_update_stolen_time(vcpu, true);
>> +
>> +	if (err)
>> +		ret = SMCCC_RET_NOT_SUPPORTED;
> 
> Trivial by why not
> 		return SMCCC_RET_NOT_SUPPORTED;
> 
> 	return vcpu->kvm->arch.pvtime.st_base +
> ...
> Drops the indentation a bit and puts the error handling out of
> line which is slightly nicer to read (to my eyes).

Yes that's a nice change - drops the extra "ret" variable too.

Thanks,

Steve