Re: [PATCH v2 10/10] arm64: Change type of hsr, cpsr, spsr_el1 to uint64_t

[Michal Orzel <michal.orzel@xxxxxxx>]

Hi Julien,

On 27.04.2021 13:09, Julien Grall wrote:
> Hi Michal,
> 
> On 27/04/2021 10:35, Michal Orzel wrote:
>> AArch64 registers are 64bit whereas AArch32 registers
>> are 32bit or 64bit. MSR/MRS are expecting 64bit values thus
>> we should get rid of helpers READ/WRITE_SYSREG32
>> in favour of using READ/WRITE_SYSREG.
>> We should also use register_t type when reading sysregs
>> which can correspond to uint64_t or uint32_t.
>> Even though many AArch64 registers have upper 32bit reserved
>> it does not mean that they can't be widen in the future.
>>
>> Modify type of hsr, cpsr, spsr_el1 to uint64_t.
> 
> As I pointed out in v1, the access to SPSR_EL1 has been quite fragile because 
> we relied on the padding afterwards. I think this was ought to be explain in 
> the commit message because it explain why the access in the assembly code is 
> not modified.
> 
How about:
"
Modify type of hsr, cpsr, spsr_el1 to uint64_t.
Previously we relied on the padding after SPSR_EL1. As we removed the padding, 
modify the union to be 64bit
so we don't corrupt SPSR_FIQ.
No need to modify the assembly code becuase the accesses were based on 64bit 
registers as there was a 32bit padding after SPSR_EL1.
"
>>
>> Add 32bit RES0 members to structures inside hsr union.
>>
>> Remove 32bit padding in cpu_user_regs before spsr_fiq
>> as it is no longer needed due to upper union being 64bit now.
>>
>> Add 64bit padding in cpu_user_regs before spsr_el1
>> because offset of spsr_el1 must be a multiple of 8.
>>
>> Signed-off-by: Michal Orzel <michal.orzel@xxxxxxx>
>> diff --git a/xen/include/asm-arm/hsr.h b/xen/include/asm-arm/hsr.h
>> index 29d4531f40..fb4a3b1274 100644
>> --- a/xen/include/asm-arm/hsr.h
>> +++ b/xen/include/asm-arm/hsr.h
>> @@ -16,11 +16,12 @@ enum dabt_size {
>>   };
>>     union hsr {
>> -    uint32_t bits;
>> +    register_t bits;
>>       struct {
>>           unsigned long iss:25;  /* Instruction Specific Syndrome */
>>           unsigned long len:1;   /* Instruction length */
>>           unsigned long ec:6;    /* Exception Class */
>> +        unsigned long _res0:32;
> 
> Sorry I wasn't clear in my original comment, what I meant I would rather not 
> add this field (and also the _res0) because they are not strictly necessary.
> 
Ok I'll remove _res0 members. But bits can be of type register_t, right?
>> diff --git a/xen/include/public/arch-arm.h b/xen/include/public/arch-arm.h
>> index 713fd65317..c49bce2983 100644
>> --- 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 */
>> +    register_t cpsr;                            /* SPSR_EL2 */
> 
> You can't use register_t here because this is a public header (we don't 
> export register_t) and the header should be bitness agnostic.
> 
> Also, because this is a public header, you ought to explain why breaking the 
> ABI is fine.
> 
> In this case, this is an ABI between the tools and this is not stable. 
> However, we would still need to bump XEN_DOMCTL_INTERFACE_VERSION as I think 
> this wasn't done for this development cycle.
> 
> Of course, this will also need a suitable mention in the commit message (I 
> can help with that).
> 
Ok so I'll increment XEN_DOMCTL_INTERFACE_VERSION and write in commit msg:
"
Change type of cpsr to uint64_t in the public outside interface 
"public/arch-arm.h" to allow ABI compatibility between 32bit and 64bit.
Increment XEN_DOMCTL_INTERFACE_VERSION.
"
>>         union {
>> -        uint32_t spsr_el1;       /* AArch64 */
>> +        uint64_t spsr_el1;       /* AArch64 */
>>           uint32_t spsr_svc;       /* AArch32 */
>>       };
>>   diff --git a/xen/include/public/vm_event.h b/xen/include/public/vm_event.h
>> index 36135ba4f1..ad3d141fe8 100644
>> --- a/xen/include/public/vm_event.h
>> +++ b/xen/include/public/vm_event.h
>> @@ -266,8 +266,12 @@ struct vm_event_regs_arm {
>>       uint64_t ttbr1;
>>       uint64_t ttbcr;
>>       uint64_t pc;
>> +#ifdef CONFIG_ARM_32
>>       uint32_t cpsr;
>>       uint32_t _pad;
>> +#else
>> +    uint64_t cpsr;
>> +#endif
> 
> CONFIG_ARM_32 is not defined for public header. They also should be bitness 
> agnostic. So cpsr should always be uint64_t.
> 
> Also, similar to public/arch-arm.h, this is not a stable ABI but you will 
> need to bump VM_EVENT_INTERFACE_VERSION if this hasn't been done for this 
> development cycle.
> 
Ok so I will change type of cpsr here to uint64_t, increment 
VM_EVENT_INTERFACE_VERSION and write in commit msg:
"
Change type of cpsr to uint64_t in the public outside interface 
"public/vm_event.h" to allow ABI compatibility between 32bit and 64bit.
Increment VM_EVENT_INTERFACE_VERSION.
"
Ok?
> Cheers,
> 
Cheers,
Michal