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

On Mon, May 30, 2016 at 2:20 PM, Julien Grall <julien.grall@xxxxxxx> 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@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.

> I already mentioned it on a previous version, and it seems to have been
> dropped from the commit message.
> Anyway, we should avoid to be stingy with comments. They are helpful for
> anyone interested by the vm_event subsystem and even you in few years if you
> decide to modify this code.

Certainly, I would be happy to put the above description into a comment here.

>>> I'm also puzzled by fp and lr - I'm not aware of registers of those
>>> names (and gas also doesn't accept them afaict).
>> Not sure why but Xen names x29 fp and x30 lr. See
>> include/asm-arm/arm64/processor.h.
> They are not officially called "lr" and "fp" in the ARM ARM. However the
> AAPCS64 [1] (5.1.1) defines x29 and x30 as special registers holding resp.
> "lr" and "fp".
> The convention is used internally in Xen for convenience. However, the
> public header uses x29 and x30 (see include/public/arch-arm.h).
> I would prefer if you use the official name in the public headers.

Sounds good to me.


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