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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 31.05.16 at 10:06, <rcojocaru@xxxxxxxxxxxxxxx> wrote:
> On 05/31/2016 10:54 AM, Jan Beulich wrote:
>>>>> On 30.05.16 at 22:37, <tamas@xxxxxxxxxxxxx> wrote:
>>> On Mon, May 30, 2016 at 2:20 PM, Julien Grall <julien.grall@xxxxxxx> wrote:
>>>> On 30/05/2016 20:47, Tamas K Lengyel wrote:
>>>>> On Mon, May 30, 2016 at 5:50 AM, Jan Beulich <JBeulich@xxxxxxxx> 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.
>> 
>> Not being an ARM maintainer I'd say "Then include just those and no
>> (other) GPRs at all, or include all GPRs." Such a "best guess" approach
>> can really only end up being wrong from some future consumer. And
>> in that consideration, please also include the aspects that lead to all
>> x86 GPRs to get included here (not to speak of even various MSR
>> values). IOW the same criteria should be applied to all architectures.
> 
> The x86 GPRS are already all included in the vm_event request:

Well, that's what I've been saying, or at least I had tried to: I
realize I've typoed the past tense of "lead" - should have been
"led". Sorry for the confusion.

Jan


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