linux-kernel - Re: [tip: x86/msr] um: Add UML version of <asm/tsc.h> to define rdtsc()

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Message-ID: <aBuqqJoHOag7ZRML@gmail.com>
Date: Wed, 7 May 2025 20:47:04 +0200
From: Ingo Molnar <mingo@...nel.org>
To: linux-kernel@...r.kernel.org, Johannes Berg <johannes.berg@...el.com>
Cc: linux-tip-commits@...r.kernel.org, kernel test robot <lkp@...el.com>,
	Johannes Berg <johannes.berg@...el.com>, x86@...nel.org
Subject: Re: [tip: x86/msr] um: Add UML version of <asm/tsc.h> to define
 rdtsc()


Johannes: this is basically an RFC, not a final commit, will add your 
Reviewed-by if it's fine to you, or will change the patch if it's not.

Thanks,

	Ingo

* tip-bot2 for Ingo Molnar <tip-bot2@...utronix.de> wrote:

> The following commit has been merged into the x86/msr branch of tip:
> 
> Commit-ID:     24b58adaa7508d9d2cdb6bca44803954baf24459
> Gitweb:        https://git.kernel.org/tip/24b58adaa7508d9d2cdb6bca44803954baf24459
> Author:        Ingo Molnar <mingo@...nel.org>
> AuthorDate:    Wed, 07 May 2025 20:18:22 +02:00
> Committer:     Ingo Molnar <mingo@...nel.org>
> CommitterDate: Wed, 07 May 2025 20:30:39 +02:00
> 
> um: Add UML version of <asm/tsc.h> to define rdtsc()
> 
> In the x86 tree rdtsc() methods got moved out of <asm/msr.h>, but this
> broke UML, as the x86 version of <asm/tsc.h> cannot be used by UML as-is:
> 
> 	  CC [M]  drivers/accel/habanalabs/common/habanalabs_ioctl.o
> 	In file included from drivers/accel/habanalabs/common/habanalabs_ioctl.c:20:
> 	./arch/x86/include/asm/tsc.h:70:28: error: conflicting types for ‘cycles_t’; have ‘long long unsigned int’
> 	   70 | typedef unsigned long long cycles_t;
> 	      |                            ^~~~~~~~
> 	In file included from ./arch/um/include/asm/timex.h:7,
> 	                 from ./include/linux/timex.h:67,
> 	                 from ./include/linux/time32.h:13,
> 	                 from ./include/linux/time.h:60,
> 	                 from ./include/linux/skbuff.h:15,
> 	                 from ./include/linux/if_ether.h:19,
> 	                 from ./include/linux/habanalabs/cpucp_if.h:12,
> 	                 from drivers/accel/habanalabs/common/habanalabs.h:11,
> 	                 from drivers/accel/habanalabs/common/habanalabs_ioctl.c:11:
> 	./include/asm-generic/timex.h:8:23: note: previous declaration of ‘cycles_t’ with type ‘cycles_t’ {aka ‘long unsigned int’}
> 	    8 | typedef unsigned long cycles_t;
> 	      |                       ^~~~~~~~
> 
> To resolve these kinds of problems and to allow <asm/tsc.h> to be included on UML,
> add a simplified version of <asm/tsc.h>, which only adds the rdtsc() definition.
> 
> Reported-by: kernel test robot <lkp@...el.com>
> Signed-off-by: Ingo Molnar <mingo@...nel.org>
> Cc: Johannes Berg <johannes.berg@...el.com>
> Link: https://lore.kernel.org/r/202505080003.0t7ewxGp-lkp@intel.com
> ---
>  arch/um/include/asm/tsc.h | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
>  create mode 100644 arch/um/include/asm/tsc.h
> 
> diff --git a/arch/um/include/asm/tsc.h b/arch/um/include/asm/tsc.h
> new file mode 100644
> index 0000000..a52b0e4
> --- /dev/null
> +++ b/arch/um/include/asm/tsc.h
> @@ -0,0 +1,25 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_UM_TSC_H
> +#define _ASM_UM_TSC_H
> +
> +#include <asm/asm.h>
> +
> +/**
> + * rdtsc() - returns the current TSC without ordering constraints
> + *
> + * rdtsc() returns the result of RDTSC as a 64-bit integer.  The
> + * only ordering constraint it supplies is the ordering implied by
> + * "asm volatile": it will put the RDTSC in the place you expect.  The
> + * CPU can and will speculatively execute that RDTSC, though, so the
> + * results can be non-monotonic if compared on different CPUs.
> + */
> +static __always_inline u64 rdtsc(void)
> +{
> +	EAX_EDX_DECLARE_ARGS(val, low, high);
> +
> +	asm volatile("rdtsc" : EAX_EDX_RET(val, low, high));
> +
> +	return EAX_EDX_VAL(val, low, high);
> +}
> +
> +#endif /* _ASM_UM_TSC_H */