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

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

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:

133 struct vm_event_regs_x86 {
134     uint64_t rax;
135     uint64_t rcx;
136     uint64_t rdx;
137     uint64_t rbx;
138     uint64_t rsp;
139     uint64_t rbp;
140     uint64_t rsi;
141     uint64_t rdi;


Thanks,
Razvan

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