Re: [Xen-devel] [PATCH v8 4/6] arm/vm_event: get/set registers

[Tamas K Lengyel <tamas.lengyel@xxxxxxxxxxxx>]

On Wed, Jul 6, 2016 at 3:12 PM, Julien Grall <julien.grall@xxxxxxx> wrote:
>
>
> On 06/07/2016 20:23, Tamas K Lengyel wrote:
>>
>> On Wed, Jul 6, 2016 at 1:43 AM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
>>>>>>
>>>>>> On 05.07.16 at 20:37, <tamas.lengyel@xxxxxxxxxxxx> 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 x11;
>>>> +    uint64_t x12;
>>>> +    uint64_t x13;
>>>> +    uint64_t x14;
>>>> +    uint64_t x15;
>>>> +    uint64_t x16;
>>>> +    uint64_t x17;
>>>> +    uint64_t x18;
>>>> +    uint64_t x19;
>>>> +    uint64_t x20;
>>>> +    uint64_t x21;
>>>> +    uint64_t x22;
>>>> +    uint64_t x23;
>>>> +    uint64_t x24;
>>>> +    uint64_t x25;
>>>> +    uint64_t x26;
>>>> +    uint64_t x27;
>>>> +    uint64_t x28;
>>>> +    uint64_t x29;
>>>> +    uint64_t x30;
>>>> +    uint64_t pc;
>>>> +};
>>>
>>>
>>> Isn't the stack pointer a fully separate register in aarch64? Not
>>> making available something as essential as that seems wrong to
>>> me.
>>>
>>
>> The register is available for access already, so unless there is an
>> actual use-case that requires it to be transmitted through vm_event I
>> don't see the point for transmitting it. So as I mentioned in my other
>> response, I'm inclined to reduce this patch to the bare essentials my
>> use-case requires at this point and leave the extensions up for the
>> future when - and if - it will be of use. Since this patch is just an
>> optimization, if transmitting such reduced set is not acceptable for
>> some reason, I'll forgo this patch entirely.
>
>
> Here we go with again the same argument: "this is not necessary for my
> use-case". This data structure is part of the ABI between the hypervisor and
> the vm-event app, i.e modifying this structure for adding ARM64/ARM
> registers will result to an incompatibility with a previous version of the
> hypervisor. Better to do it now than in a couple of years when vm-event will
> have more users. I agree that it is time consuming to get an ABI correct,
> but it will save users to recompile/ship another version of their vm-event
> app because of this incompatibility.
>
> As mentioned in a previous thread, the main use-case for trapping an SMC is
> emulating the call, hence a vm-event app would want to have access to the
> general-purpose registers. And yes, I know that your use-case is different
> and does not require those registers, I already expressed my concerns about
> it.
>
> Now, if you drop this patch, how will you retrieve the vCPU register? I
> guess via DOMCTL_getvcpucontext? If so, if the vCPU has not been paused, you
> will get a context which is different compare to the time the vm-event has
> occurred. And yes, I know that in your use-case the vCPU is paused. This
> call will always be more expensive than passing the registers with event.

Ack but as right now this patch is just an optimization with no
use-case where it is required, so I'll just drop it from my series.

>
> Anyway, I really don't think we ask for something really difficult to
> accomplish.
>

That may be so and it can be revisited in the future when and if it
will be a hard requirement.

Thanks,
Tamas

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