Re: [PATCH v4 5/8] arm/vm_event: get/set registers

[Tamas K Lengyel <tamas@tklengyel.com>]

On Mon, May 30, 2016 at 3:35 PM, Julien Grall <julien.grall@arm.com> wrote:
>
>
> On 30/05/2016 21:37, Tamas K Lengyel wrote:
>>
>> On Mon, May 30, 2016 at 2:20 PM, Julien Grall <julien.grall@arm.com>
>> wrote:
>>>
>>> Hi Tamas,
>>>
>>> On 30/05/2016 20:47, Tamas K Lengyel wrote:
>>>>
>>>>
>>>> On Mon, May 30, 2016 at 5:50 AM, Jan Beulich <JBeulich@suse.com> wrote:
>>>>>>
>>>>>>
>>>>>> +struct vm_event_regs_arm64 {
>>>>>> +    uint64_t x0;
>>>>>> +    uint64_t x1;
>>>>>> +    uint64_t x2;
>>>>>> +    uint64_t x3;
>>>>>> +    uint64_t x4;
>>>>>> +    uint64_t x5;
>>>>>> +    uint64_t x6;
>>>>>> +    uint64_t x7;
>>>>>> +    uint64_t x8;
>>>>>> +    uint64_t x9;
>>>>>> +    uint64_t x10;
>>>>>> +    uint64_t x16;
>>>>>> +    uint64_t lr;
>>>>>> +    uint64_t fp;
>>>>>> +    uint64_t pc;
>>>>>> +    uint64_t sp_el0;
>>>>>> +    uint64_t sp_el1;
>>>>>> +    uint32_t spsr_el1;
>>>>>> +    uint32_t _pad;
>>>>>> +};
>>>>>
>>>>>
>>>>>
>>>>> My ARM knowledge is certainly quite limited, but the incomplete set
>>>>> of GPRs here is quite obvious: Is there a reason to not make all of
>>>>> them available here? And if there is, could the criteria of which
>>>>> registers got put here please be documented in a comment (so that
>>>>> the next person noticing the incomplete set won't ask again)?
>>>>
>>>>
>>>>
>>>> There already is a comment in place that explains why we are not
>>>> sending the full set of registers here.
>>>
>>>
>>>
>>> Your comment only says:
>>> "Using custom vCPU structs (i.e. not hvm_hw_cpu) for both x86 and ARM
>>> so as to not fill the vm_event ring buffer too quickly." it does not
>>> explain
>>> the criteria of which registers got put here.
>>
>>
>> Well, as we discussed it in the previous revision, there is no
>> hard-set rule of what can and cannot be transmitted here. The only
>> thing to keep in mind is to not grow this struct to be too large. The
>> registers sent right now represent a "best guess" of what may be
>> useful for performance-sensitive vm_event applications on ARM. It can
>> be adjusted in the future if applications require other registers.
>> Right now there are no applications at all in this space so we don't
>> have any specifications to rely on for making this selection. I'm
>> happy to make adjustments to the selection if others have a better
>> idea what to send, the only registers I certainly find very useful are
>> TTBR0/1, cpsr and pc at this time.
>
>
> Please log it in the commit message and the code. If someone emitted
> multiple time the same concern on previous version, it likely means that
> your commit message was not clear enough and should be updated.
>
> The number of patch to review on Xen-devel is very consequence, so we cannot
> really afford to spend a lot of time digging into previous threads. As a
> maintainer of a subsystem, you should be aware of that.
>
> We are trying, at least on ARM, to get as much details as possible in the
> commit message and document any possible unclear code to help developer
> understanding why it has been done like that. It also helps us (the
> reviewers and maintainers) to give useful advice later on.
>

Of course, thanks.

Tamas

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