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charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-riscv" Errors-To: linux-riscv-bounces+linux-riscv=archiver.kernel.org@lists.infradead.org On 26. 6. 25. 12:49, Andrew Jones wrote:> On Thu, Jun 26, 2025 at 11:34:21AM +0200, Andrew Jones wrote: >> On Thu, Jun 26, 2025 at 11:21:10AM +0200, Andrew Jones wrote: >>> On Wed, Jun 25, 2025 at 04:21:01PM +0200, Aleksa Paunovic via B4 Relay wrote: >>>> From: Aleksa Paunovic >>>> >>>> Use the hwprobe syscall to decide which PAUSE instruction to execute in >>>> userspace code. >>>> >>>> Signed-off-by: Aleksa Paunovic >>>> --- >>>> tools/arch/riscv/include/asm/vdso/processor.h | 27 +++++++++++++++++---------- >>>> 1 file changed, 17 insertions(+), 10 deletions(-) >>>> >>>> diff --git a/tools/arch/riscv/include/asm/vdso/processor.h b/tools/arch/riscv/include/asm/vdso/processor.h >>>> index 662aca03984817f9c69186658b19e9dad9e4771c..027219a486b7b93814888190f8224af29498707c 100644 >>>> --- a/tools/arch/riscv/include/asm/vdso/processor.h >>>> +++ b/tools/arch/riscv/include/asm/vdso/processor.h >>>> @@ -4,26 +4,33 @@ >>>> >>>> #ifndef __ASSEMBLY__ >>>> >>>> +#include >>>> +#include >>>> +#include >>>> #include >>>> >>>> static inline void cpu_relax(void) >>>> { >>>> + struct riscv_hwprobe pair; >>>> + bool has_mipspause; >>>> #ifdef __riscv_muldiv >>>> int dummy; >>>> /* In lieu of a halt instruction, induce a long-latency stall. */ >>>> __asm__ __volatile__ ("div %0, %0, zero" : "=r" (dummy)); >>>> #endif >>>> >>>> -#ifdef CONFIG_TOOLCHAIN_HAS_ZIHINTPAUSE >>>> - /* >>>> - * Reduce instruction retirement. >>>> - * This assumes the PC changes. >>>> - */ >>>> - __asm__ __volatile__ ("pause"); >>>> -#else >>>> - /* Encoding of the pause instruction */ >>>> - __asm__ __volatile__ (".4byte 0x100000F"); >>>> -#endif >>>> + pair.key = RISCV_HWPROBE_KEY_VENDOR_EXT_MIPS_0; >>>> + __riscv_hwprobe(&pair, 1, 0, NULL, 0); >>>> + has_mipspause = pair.value & RISCV_HWPROBE_VENDOR_EXT_XMIPSEXECTL; >>>> + >>>> + if (has_mipspause) { >>>> + /* Encoding of the mips pause instruction */ >>>> + __asm__ __volatile__(".4byte 0x00501013"); >>>> + } else { >>>> + /* Encoding of the pause instruction */ >>>> + __asm__ __volatile__(".4byte 0x100000F"); >>>> + } >>>> + >>> >>> cpu_relax() is used in places where we cannot afford the overhead nor call >>> arbitrary functions which may take locks, etc. We've even had trouble >>> using a static key here in the past since this is inlined and it bloated >>> the size too much. You'll need to use ALTERNATIVE(). >> >> Oh, I see now that the next patch is handling the kernel cpu_relax with >> ALTERNATIVE and this was just the tools cpu_relax. We don't want to make >> a syscall inside cpu_relax though either, since it gets called in loops. > > (Another follow up to myself...) > > I guess with the vdso cached result it should only be a handful of > instructions, but it still seems odd to embed a call in cpu_relax. > Hi Andrew, Thank you for your comments! > Thanks, > drew > >> It'd be better to just call the standard pause (0x100000F) even if it >> does nothing. Or maybe there's some define that can be added/used to >> select the correct instruction? >> We did try using an ifdef/else in v3, but since that would have to be marked non-portable, we decided to go with a hwprobe call. Since the MIPS pause should behave as a nop on other CPUs, would leaving both the standard pause and the MIPS pause calls be an acceptable solution? That said, I am not sure how this would behave on future MIPS CPUs in case they support both encodings. Best regards, Aleksa >> Thanks, >> drew _______________________________________________ linux-riscv mailing list linux-riscv@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-riscv