Re: [PATCH v3 10/10] arm64: Change type of hsr, cpsr, spsr_el1 to uint64_t

[Michal Orzel <michal.orzel@arm.com>]

On 21.05.2021 09:07, Jan Beulich wrote:
> On 21.05.2021 08:33, Michal Orzel wrote:
>> On 17.05.2021 18:03, Julien Grall wrote:
>>> On 17/05/2021 08:01, Jan Beulich wrote:
>>>> On 12.05.2021 19:59, Julien Grall wrote:
>>>>> Hi,
>>>>>
>>>>> On 11/05/2021 07:37, Michal Orzel wrote:
>>>>>> On 05.05.2021 10:00, Jan Beulich wrote:
>>>>>>> On 05.05.2021 09:43, Michal Orzel wrote:
>>>>>>>> --- a/xen/include/public/arch-arm.h
>>>>>>>> +++ b/xen/include/public/arch-arm.h
>>>>>>>> @@ -267,10 +267,10 @@ struct vcpu_guest_core_regs
>>>>>>>>           /* Return address and mode */
>>>>>>>>        __DECL_REG(pc64,         pc32);             /* ELR_EL2 */
>>>>>>>> -    uint32_t cpsr;                              /* SPSR_EL2 */
>>>>>>>> +    uint64_t cpsr;                              /* SPSR_EL2 */
>>>>>>>>           union {
>>>>>>>> -        uint32_t spsr_el1;       /* AArch64 */
>>>>>>>> +        uint64_t spsr_el1;       /* AArch64 */
>>>>>>>>            uint32_t spsr_svc;       /* AArch32 */
>>>>>>>>        };
>>>>>>>
>>>>>>> This change affects, besides domctl, also default_initialise_vcpu(),
>>>>>>> which Arm's arch_initialise_vcpu() calls. I realize do_arm_vcpu_op()
>>>>>>> only allows two unrelated VCPUOP_* to pass, but then I don't
>>>>>>> understand why arch_initialise_vcpu() doesn't simply return e.g.
>>>>>>> -EOPNOTSUPP. Hence I suspect I'm missing something.
>>>>>
>>>>> I think it is just an overlooked when reviewing the following commit:
>>>>>
>>>>> commit 192df6f9122ddebc21d0a632c10da3453aeee1c2
>>>>> Author: Roger Pau Monné <roger.pau@citrix.com>
>>>>> Date:   Tue Dec 15 14:12:32 2015 +0100
>>>>>
>>>>>       x86: allow HVM guests to use hypercalls to bring up vCPUs
>>>>>
>>>>>       Allow the usage of the VCPUOP_initialise, VCPUOP_up, VCPUOP_down,
>>>>>       VCPUOP_is_up, VCPUOP_get_physid and VCPUOP_send_nmi hypercalls from HVM
>>>>>       guests.
>>>>>
>>>>>       This patch introduces a new structure (vcpu_hvm_context) that
>>>>> should be used
>>>>>       in conjuction with the VCPUOP_initialise hypercall in order to
>>>>> initialize
>>>>>       vCPUs for HVM guests.
>>>>>
>>>>>       Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
>>>>>       Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
>>>>>       Reviewed-by: Jan Beulich <jbeulich@suse.com>
>>>>>       Acked-by: Ian Campbell <ian.campbell@citrix.com>
>>>>>
>>>>> On Arm, the structure vcpu_guest_context is not exposed outside of Xen
>>>>> and the tools. Interestingly vcpu_guest_core_regs is but it should only
>>>>> be used within vcpu_guest_context.
>>>>>
>>>>> So as this is not used by stable ABI, it is fine to break it.
>>>>>
>>>>>>>
>>>>>> I agree that do_arm_vcpu_op only allows two VCPUOP* to pass and
>>>>>> arch_initialise_vcpu being called in case of VCPUOP_initialise
>>>>>> has no sense as VCPUOP_initialise is not supported on arm.
>>>>>> It makes sense that it should return -EOPNOTSUPP.
>>>>>> However do_arm_vcpu_op will not accept VCPUOP_initialise and will return
>>>>>> -EINVAL. So arch_initialise_vcpu for arm will not be called.
>>>>>> Do you think that changing this behaviour so that arch_initialise_vcpu returns
>>>>>> -EOPNOTSUPP should be part of this patch?
>>>>>
>>>>> I think this change is unrelated. So it should be handled in a follow-up
>>>>> patch.
>>>>
>>>> My only difference in viewing this is that I'd say the adjustment
>>>> would better be a prereq patch to this one, such that the one here
>>>> ends up being more obviously correct.
>>>
>>> The function is already not reachable so I felt it was unfair to require the clean-up for merging this code.
>>>
>>>> Also, if the function is
>>>> indeed not meant to be reachable, besides making it return
>>>> -EOPNOTSUPP (or alike) it should probably also have
>>>> ASSERT_UNREACHABLE() added.
>>>
>>> +1 on the idea.
>>>
>>> Cheers,
>>>
>> FWICS, all the discussion is about creating the next patch fixing the VCPUOP_initialise function.
>> Is there anything left to do in this patch or are you going to ack it?
> 
> Afaic I'd find it quite helpful if that other patch was a prereq to this
> one, making more obvious that the change here is not going to break
> anything. But it's Arm stuff, so Arm folks get the final say anyway.
This change is not going to break anything as the new patch is going to mainly add ASSERT_UNREACHABLE into VCPUOP_initialise which means it'll be a clean-up patch.
Also the problem was not introduced by this patch so I think it should be merged.
> 
> Jan
> 

So what is the final say from Arm folks :) ?