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Re: [PATCH v2 3/6] xen/riscv: introduce function for physical offset calculation
- To: Oleksii <oleksii.kurochko@xxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Fri, 7 Jul 2023 11:35:25 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Bob Eshleman <bobbyeshleman@xxxxxxxxx>, Alistair Francis <alistair.francis@xxxxxxx>, Connor Davis <connojdavis@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Fri, 07 Jul 2023 09:35:36 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 07.07.2023 11:12, Oleksii wrote:
> On Thu, 2023-07-06 at 13:18 +0200, Jan Beulich wrote:
>> On 19.06.2023 15:34, Oleksii Kurochko wrote:
>>> --- a/xen/arch/riscv/riscv64/head.S
>>> +++ b/xen/arch/riscv/riscv64/head.S
>>> @@ -29,6 +29,8 @@ ENTRY(start)
>>>
>>> jal reset_stack
>>>
>>> + jal calc_phys_offset
>>> +
>>> tail start_xen
>>>
>>> .section .text, "ax", %progbits
>>
>> Since you call a C function, the code to save/restore a0/a1 needs to
>> move here (from patch 4).
> Thanks. It makes sense.
> It would be better to move save/restore a0/a1 ( from patch 4 )code
> here.
>
> The only one reason I didn't do that before that calc_phys_offset
> doesn't touch that and it is guaranteed that it will not ( as it
> doesn't have arguments )
How does a function not having parameters guarantee that registers
used for parameter passing aren't touched? Inside a function, the
compiler is free to use argument-passing registers just like other
temporary ones; their values don't need preserving, from all I know
(otherwise the RISC-V ABI would be different to all other ABIs I
know of).
Jan
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