Re: [PATCH 1/2] kvm/arm64: Rename HSR to ESR

From: Gavin Shan <gshan@redhat.com>
To: Marc Zyngier <maz@misterjones.org>, Mark Rutland <mark.rutland@arm.com>
Cc: catalin.marinas@arm.com, will@kernel.org,
	kvmarm@lists.cs.columbia.edu,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [PATCH 1/2] kvm/arm64: Rename HSR to ESR
Date: Tue, 30 Jun 2020 09:14:44 +1000	[thread overview]
Message-ID: <1cc51ce6-0e94-03e5-85e1-66d074bb50f4@redhat.com> (raw)
In-Reply-To: <9080c56e3f54d7661a57c5e2a771363f@misterjones.org>

On 6/30/20 3:00 AM, Marc Zyngier wrote:
> On 2020-06-29 11:32, Mark Rutland wrote:
>> On Mon, Jun 29, 2020 at 07:18:40PM +1000, Gavin Shan wrote:
>>> kvm/arm32 isn't supported since commit 541ad0150ca4 ("arm: Remove
>>> 32bit KVM host support"). So HSR isn't meaningful since then. This
>>> renames HSR to ESR accordingly. This shouldn't cause any functional
>>> changes:
>>>
>>>    * Rename kvm_vcpu_get_hsr() to kvm_vcpu_get_esr() to make the
>>>      function names self-explanatory.
>>>    * Rename variables from @hsr to @esr to make them self-explanatory.
>>>
>>> Signed-off-by: Gavin Shan <gshan@redhat.com>
>>
>> At a high-level, I agree that we should move to the `esr` naming to
>> match the architecture and minimize surprise. However, I think there are
>> some ABI changes here, which *are* funcitonal changes, and those need to
>> be avoided.
>>
>> [...]
>>
>>> diff --git a/arch/arm64/include/uapi/asm/kvm.h b/arch/arm64/include/uapi/asm/kvm.h
>>> index ba85bb23f060..d54345573a88 100644
>>> --- a/arch/arm64/include/uapi/asm/kvm.h
>>> +++ b/arch/arm64/include/uapi/asm/kvm.h
>>> @@ -140,7 +140,7 @@ struct kvm_guest_debug_arch {
>>>  };
>>>
>>>  struct kvm_debug_exit_arch {
>>> -    __u32 hsr;
>>> +    __u32 esr;
>>>      __u64 far;    /* used for watchpoints */
>>>  };
>>
>> This is userspace ABI, and changing this *will* break userspace. This
>> *is* a functional change.
>>
>> NAK to this specifically. At best these should be a comment here that
>> this is naming is legacym but must stay for ABI reasons.
>>
>> [...]
>>
>>> diff --git a/arch/arm64/kvm/trace_arm.h b/arch/arm64/kvm/trace_arm.h
>>> index 4c71270cc097..ee4f691b16ff 100644
>>> --- a/arch/arm64/kvm/trace_arm.h
>>> +++ b/arch/arm64/kvm/trace_arm.h
>>> @@ -42,7 +42,7 @@ TRACE_EVENT(kvm_exit,
>>>          __entry->vcpu_pc        = vcpu_pc;
>>>      ),
>>>
>>> -    TP_printk("%s: HSR_EC: 0x%04x (%s), PC: 0x%08lx",
>>> +    TP_printk("%s: ESR_EC: 0x%04x (%s), PC: 0x%08lx",
>>>            __print_symbolic(__entry->ret, kvm_arm_exception_type),
>>>            __entry->esr_ec,
>>>            __print_symbolic(__entry->esr_ec, kvm_arm_exception_class),
>>
>> Likewise, isn't all the tracepoint format stuff ABI? I'm not comfortable
>> that we can change this.
> 
> Tracepoints are ABI, and they cannot change. As it is, this patch
> isn't acceptable (the worse offender being the uapi change though).
> 

Yes, I was reluctant to make the changes regarding the uapi/tracepoint,
which is part of the ABI. I will drop the changes in v2.

Thanks,
Gavin

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