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Re: [PATCH v2 33/39] xen/riscv: add minimal stuff to asm/processor.h to build full Xen
- To: Oleksii <oleksii.kurochko@xxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Mon, 18 Dec 2023 12:38:21 +0100
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- Cc: Alistair Francis <alistair.francis@xxxxxxx>, Bob Eshleman <bobbyeshleman@xxxxxxxxx>, Connor Davis <connojdavis@xxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Mon, 18 Dec 2023 11:38:29 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 18.12.2023 11:49, Oleksii wrote:
> On Thu, 2023-12-14 at 17:04 +0100, Jan Beulich wrote:
>> On 24.11.2023 11:30, Oleksii Kurochko wrote:
>>> +static inline void cpu_relax(void)
>>> +{
>>> + int dummy;
>>> + /* In lieu of a halt instruction, induce a long-latency
>>> stall. */
>>> + __asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy));
>>
>> Any reason for this, when Arm's is just barrier(), and apparently
>> they got
>> away with this quite fine? Also isn't this causing a division by
>> zero,
>> which I'd expect to cause some kind of exception? (Terminology-wise
>> I'm of
>> course biased by x86, where "halt instruction" wouldn't be suitable
>> to use
>> here. But if that terminology is fine on RISC-V, then obviously no
>> objection.)
> It was based on Linux kernel code:
> https://elixir.bootlin.com/linux/latest/source/arch/riscv/include/asm/vdso/processor.h#L9
>
> But looks I missed barrier()...
> Probably it will be better update cpu_relax() to:
>
> /* Encoding of the pause instruction */
> __asm__ __volatile__ (".4byte 0x100000F");
>
> barrier();
But definitely without .4byte, which defines a piece of data. If for
whatever reason you don't want to use "pause" directly, please use
.insn.
Jan
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