linux-kernel - Re: [PATCH v5 05/10] KVM: arm64: Support stolen time reporting via shared structure

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Message-ID: <b107c1ca-6804-dc47-af25-fcd0b201472f@arm.com>
Date:   Fri, 4 Oct 2019 10:13:40 +0100
From:   Steven Price <steven.price@....com>
To:     Andrew Jones <drjones@...hat.com>
Cc:     Marc Zyngier <maz@...nel.org>, Will Deacon <will@...nel.org>,
        linux-arm-kernel@...ts.infradead.org, kvmarm@...ts.cs.columbia.edu,
        Catalin Marinas <catalin.marinas@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Russell King <linux@...linux.org.uk>,
        James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.com>,
        Suzuki K Pouloze <suzuki.poulose@....com>,
        Mark Rutland <mark.rutland@....com>, kvm@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 05/10] KVM: arm64: Support stolen time reporting via
 shared structure

On 04/10/2019 08:03, Andrew Jones wrote:
> On Thu, Oct 03, 2019 at 03:22:35PM +0200, Andrew Jones wrote:
>> On Wed, Oct 02, 2019 at 03:50:32PM +0100, Steven Price wrote:
>>> +int kvm_update_stolen_time(struct kvm_vcpu *vcpu, bool init)
>>> +{
>>> +	struct kvm *kvm = vcpu->kvm;
>>> +	u64 steal;
>>> +	u64 steal_le;
>>> +	u64 offset;
>>> +	int idx;
>>> +	u64 base = vcpu->arch.steal.base;
>>> +
>>> +	if (base == GPA_INVALID)
>>> +		return -ENOTSUPP;
>>> +
>>> +	/* Let's do the local bookkeeping */
>>> +	steal = vcpu->arch.steal.steal;
>>> +	steal += current->sched_info.run_delay - vcpu->arch.steal.last_steal;
>>> +	vcpu->arch.steal.last_steal = current->sched_info.run_delay;
>>> +	vcpu->arch.steal.steal = steal;
>>> +
>>> +	steal_le = cpu_to_le64(steal);
>>
>> Agreeing on a byte order for this interface makes sense, but I don't see
>> it documented anywhere. Is this an SMCCC thing? Because I skimmed some
>> of those specs and other users too but didn't see anything obvious. Anyway
>> even if everybody but me knows that all data returned from SMCCC calls
>> should be LE, it might be nice to document that in the pvtime doc.

A very good point - I'll document this in the Linux document and feed
that back for DEN0057A.

> 
> I have another [potentially dumb] SMCCC byte order question. If we need
> to worry about using LE for the members of this structure, then why don't
> we need to worry about the actual return values of the SMCCC calls? Like
> the IPA of the structure?

The SMCCC calls pass values in registers. It's only when reading/writing
these values from/to memory that the endianness actually has any meaning.

Steve