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

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Wed, Jun 1, 2016 at 1:38 PM, Julien Grall <julien.grall@xxxxxxx> wrote:
> Hi Tamas,
>
>
> On 01/06/16 19:21, Tamas K Lengyel wrote:
>>
>> On Wed, Jun 1, 2016 at 5:24 AM, Julien Grall <julien.grall@xxxxxxx> wrote:
>>>
>>> Hi,
>>>
>>>
>>> On 01/06/16 09:41, Jan Beulich wrote:
>>>>>>>
>>>>>>>
>>>>>>> On 31.05.16 at 18:28, <tamas@xxxxxxxxxxxxx> wrote:
>>>>>
>>>>>
>>>>> On May 31, 2016 01:48, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
>>>>>>
>>>>>>
>>>>>>
>>>>>>>>> On 30.05.16 at 21:47, <tamas@xxxxxxxxxxxxx> wrote:
>>>>>>>
>>>>>>>
>>>>>>> On Mon, May 30, 2016 at 5:50 AM, Jan Beulich <JBeulich@xxxxxxxx>
>>>>>>> wrote:
>>>>>>>>>>>
>>>>>>>>>>>
>>>>>>>>>>> On 30.05.16 at 00:37, <tamas@xxxxxxxxxxxxx> wrote:
>>>>>>>>>
>>>>>>>>>
>>>>>>>>> +struct vm_event_regs_arm32 {
>>>>>>>>> +    uint32_t r0_usr;
>>>>>>>>> +    uint32_t r1_usr;
>>>>>>>>> +    uint32_t r2_usr;
>>>>>>>>> +    uint32_t r3_usr;
>>>>>>>>> +    uint32_t r4_usr;
>>>>>>>>> +    uint32_t r5_usr;
>>>>>>>>> +    uint32_t r6_usr;
>>>>>>>>> +    uint32_t r7_usr;
>>>>>>>>> +    uint32_t r8_usr;
>>>>>>>>> +    uint32_t r9_usr;
>>>>>>>>> +    uint32_t r10_usr;
>>>>>>>>> +    uint32_t r12_usr;
>>>>>>>>> +    uint32_t lr_usr;
>>>>>>>>> +    uint32_t fp;
>>>>>>>>> +    uint32_t pc;
>>>>>>>>> +    uint32_t sp_usr;
>>>>>>>>> +    uint32_t sp_svc;
>>>>>>>>> +    uint32_t spsr_svc;
>>>>>>>>> +};
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>> It would seem more natural for the "ordinary" registers to be
>>>>>>>> arranged in the numerical sequence, i.e. fp, r12, sp, lr, pc.
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> Not sure I follow.
>>>>>>
>>>>>>
>>>>>>
>>>>>> For one it is quite natural for someone looking at a sequence of
>>>>>> register values to assume / expect them to be in natural order.
>>>>>> And then, having them in natural (numeric) order allows e.g.
>>>>>> extracting the register identifying bits from an instruction to use
>>>>>> them as an array index into (part of) this structure.
>>>>>>
>>>>>> (For some background: I've been bitten a number of times by
>>>>>> people sorting x86 registers alphabetically instead or naturally,
>>>>>> i.e. EAX, EBX, ECX, EDX instead of EAX, ECX, EDX, EBX).
>>>>>
>>>>>
>>>>>
>>>>> I see, however I believe that would be a very careless use of this
>>>>> struct
>>>>> from the user as the register sizes are not even necessarily match the
>>>>> architecture. For example we only define the 64bit x86 registers, so
>>>>> trying
>>>>> to access it as an array of 32bit registers wouldn't work at all. Plus
>>>>> we
>>>>> are not doing a full set of registers, and I rather not imply that
>>>>> every
>>>>> element in the "natural sequence" is present. It may be, it may be not.
>>>>
>>>>
>>>>
>>>> Once an ABI is set in stone, and if that ABI allows for optimizations
>>>> (by consumers) like the one mentioned, I don't think this would be
>>>> careless use. The resulting code is very clearly much more efficient
>>>> than e.g. a switch() statement with a case label for each and every
>>>> register. Well, yes, I already hear the "memory is cheap and hence
>>>> code size doesn't matter" argument, but as said elsewhere quite
>>>> recently I don't buy this.
>>>
>>>
>>>
>>> I agree with Jan here.
>>>
>>> ARM64 has 31 general purposes registers (x0-x30). The switch to find a
>>> register based on the index will be quite big.
>>>
>>> If you order the register and provide all the general purposes registers
>>> (x0
>>> - x30), you will be able to replace by a single line (for instance see
>>> select_user_reg in arch/arm/traps.c).
>>
>>
>> The issue is that the x86 vm_event struct right now has 32*uint64_t
>> size. So if we would want to transmit all ARM64 GPRs + cpsr, PC and
>> TTBR0/1 we would end up growing this structure beyond what it is
>> currently. I want to avoid that as it affects both ARM32 and x86
>> introspection applications as well as now we can hold fewer events on
>> the ring. I would say it is better to avoid that then potentially
>> saving some on a switch in ARM64 introspection applications.
>
>
> The design choice made for x86 should not impact the ARM design (and
> vice-versa). There are key structures in the public interface which differ
> between x86 and ARM (see arch_vcpu_info and arch_shared_info). And this is
> fine because Xen is not meant to run an x86 guest on an ARM hypervisor.
>
> As far as I can tell, we currently support VM_EVENT_REASON_MEM_ACCESS and
> VM_EVENT_REASON_GUEST_REQUEST. So technically the structure is set in stone.
> However, we have an interface version that could be bumped, we can still
> decide what is the sensible choice.
>
> With your suggestions only a part of the general-purpose registers will be
> present in the vm_event. I understand that the ring has a limited size. If I
> counted correctly, the current size of the vm_event structure is 304 bytes.
> So 15 vm_event slots are available.
>
> If we grow the structure for ARM64, i.e 3 64-bits registers (PC, TTBR0,
> TTBR1) and 1 32-bit register (CPSR). Which mean 311 bytes, i.e 13 vm_event
> slots.
>
> If the vm event subsystem is under pressure, I admit that 2 slots could be a
> lot. However, as not all the GPRs will be available in the structure you may
> have to fetch them, through XEN_DOMCTL_getvcpucontext I guess (?). The
> impact of the introspection to the guest will be significant.
>
> I cannot tell how often this will be the case, but I can tell you a compiler
> can do anything with the register allocation. I.e it could decide to
> privilege allocation in registers x20-x30 (because big number are nicer).

Fair point.

>
> If you are still concern about the pressure on the ring page, Razvan
> suggested to support multi-ring page.

Going the multi-page ring route is a bit beyond of what I would like
to get reasonable done right now. As per Razvan's comment growing the
struct size is not that big of a concern so I'll include all ARM64
GPRs in the next revision.

Thanks,
Tamas

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