linux-kernel - Re: [PATCH v6 05/10] KVM: arm64: Support stolen time reporting via shared structure

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Date:   Mon, 21 Oct 2019 11:40:15 +0100
From:   Marc Zyngier <maz@...nel.org>
To:     Steven Price <steven.price@....com>
Cc:     Mark Rutland <mark.rutland@....com>, <kvm@...r.kernel.org>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Suzuki K Pouloze <suzuki.poulose@....com>,
        <linux-doc@...r.kernel.org>, Russell King <linux@...linux.org.uk>,
        <linux-kernel@...r.kernel.org>, James Morse <james.morse@....com>,
        Julien Thierry <julien.thierry.kdev@...il.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 v6 05/10] KVM: arm64: Support stolen time reporting via  shared structure

On 2019-10-21 11:21, Steven Price wrote:
> On 19/10/2019 12:12, Marc Zyngier wrote:
>> On Fri, 11 Oct 2019 13:59:25 +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.
>>>
>>> User space allocates memory which is placed at an IPA also 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>
>>> ---
>>>  arch/arm/include/asm/kvm_host.h   | 20 +++++++++++
>>>  arch/arm64/include/asm/kvm_host.h | 21 +++++++++++-
>>>  arch/arm64/kvm/Kconfig            |  1 +
>>>  include/linux/kvm_types.h         |  2 ++
>>>  virt/kvm/arm/arm.c                | 11 ++++++
>>>  virt/kvm/arm/hypercalls.c         |  3 ++
>>>  virt/kvm/arm/pvtime.c             | 56 
>>> +++++++++++++++++++++++++++++++
>>>  7 files changed, 113 insertions(+), 1 deletion(-)

[...]

>>> +long kvm_hypercall_stolen_time(struct kvm_vcpu *vcpu)
>>
>> Why long? If that's a base address, then it is either a phys_addr_t 
>> or
>> a gpa_t. I'd suggest you move the error check to the caller.
>
> This is a bit more tricky. It's a long because that's the declared 
> type
> of the SMCCC return in kvm_hvc_call_handler(). I can't (easily) move 
> the
> code into kvm_hvc_call_handler() because that is compiled for arm (as
> well as arm64) and we don't have the definitions for stolen time 
> there.
> The best option I could come up with is to have a dummy stub for arm 
> and
> use generic types for this function.
>
> This means we need a type which can contain both a gpa_t and the
> SMCCC_RET_NOT_SUPPORTED error code.
>
> I'm open to alternative suggestions on how to make this work.

My suggestion would be to always return a gpa_t from this function, and
change the 32bit stub for kvm_hypercall_stolen_time() to always return
GPA_INVALID.

Thanks,

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