Subject: Re: [PATCH 07/23] membarrier: Rewrite sync_core_before_usermode() and improve documentation

From: Mathieu Desnoyers <[email-masked]>
Date: Wed, 12 Jan 2022 11:11:04 -0500 (EST)

----- On Jan 8, 2022, at 11:43 AM, Andy Lutomirski [email-masked] wrote:

> The old sync_core_before_usermode() comments suggested that a
> non-icache-syncing return-to-usermode instruction is x86-specific and that
> all other architectures automatically notice cross-modified code on return
> to userspace.
> 
> This is misleading.  The incantation needed to modify code from one
> CPU and execute it on another CPU is highly architecture dependent.
> On x86, according to the SDM, one must modify the code, issue SFENCE
> if the modification was WC or nontemporal, and then issue a "serializing
> instruction" on the CPU that will execute the code.  membarrier() can do
> the latter.
> 
> On arm, arm64 and powerpc, one must flush the icache and then flush the
> pipeline on the target CPU, although the CPU manuals don't necessarily use
> this language.
> 
> So let's drop any pretense that we can have a generic way to define or
> implement membarrier's SYNC_CORE operation and instead require all
> architectures to define the helper and supply their own documentation as to
> how to use it.  This means x86, arm64, and powerpc for now.  Let's also
> rename the function from sync_core_before_usermode() to
> membarrier_sync_core_before_usermode() because the precise flushing details
> may very well be specific to membarrier, and even the concept of
> "sync_core" in the kernel is mostly an x86-ism.
> 
> (It may well be the case that, on real x86 processors, synchronizing the
> icache (which requires no action at all) and "flushing the pipeline" is
> sufficient, but trying to use this language would be confusing at best.
> LFENCE does something awfully like "flushing the pipeline", but the SDM
> does not permit LFENCE as an alternative to a "serializing instruction"
> for this purpose.)

A few comments below:

[...]

> +# On powerpc, a program can use MEMBARRIER_CMD_PRIVATE_EXPEDITED_SYNC_CORE
> +# similarly to arm64.  It would be nice if the powerpc maintainers could
> +# add a more clear explanantion.

Any thoughts from ppc maintainers ?

[...]

> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index e9da3dc71254..b47cd22b2eb1 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -17,7 +17,7 @@
> #include <linux/kprobes.h>
> #include <linux/mmu_context.h>
> #include <linux/bsearch.h>
> -#include <linux/sync_core.h>
> +#include <asm/sync_core.h>

All this churn wrt move from linux/sync_core.h to asm/sync_core.h
should probably be moved to a separate cleanup patch.

> #include <asm/text-patching.h>
> #include <asm/alternative.h>
> #include <asm/sections.h>
> diff --git a/arch/x86/kernel/cpu/mce/core.c b/arch/x86/kernel/cpu/mce/core.c
> index 193204aee880..a2529e09f620 100644
> --- a/arch/x86/kernel/cpu/mce/core.c
> +++ b/arch/x86/kernel/cpu/mce/core.c
> @@ -41,12 +41,12 @@
> #include <linux/irq_work.h>
> #include <linux/export.h>
> #include <linux/set_memory.h>
> -#include <linux/sync_core.h>
> #include <linux/task_work.h>
> #include <linux/hardirq.h>
> 
> #include <asm/intel-family.h>
> #include <asm/processor.h>
> +#include <asm/sync_core.h>
> #include <asm/traps.h>
> #include <asm/tlbflush.h>
> #include <asm/mce.h>

[...]

> diff --git a/drivers/misc/sgi-gru/grufault.c b/drivers/misc/sgi-gru/grufault.c
> index d7ef61e602ed..462c667bd6c4 100644
> --- a/drivers/misc/sgi-gru/grufault.c
> +++ b/drivers/misc/sgi-gru/grufault.c
> @@ -20,8 +20,8 @@
> #include <linux/io.h>
> #include <linux/uaccess.h>
> #include <linux/security.h>
> -#include <linux/sync_core.h>
> #include <linux/prefetch.h>
> +#include <asm/sync_core.h>
> #include "gru.h"
> #include "grutables.h"
> #include "grulib.h"
> diff --git a/drivers/misc/sgi-gru/gruhandles.c
> b/drivers/misc/sgi-gru/gruhandles.c
> index 1d75d5e540bc..c8cba1c1b00f 100644
> --- a/drivers/misc/sgi-gru/gruhandles.c
> +++ b/drivers/misc/sgi-gru/gruhandles.c
> @@ -16,7 +16,7 @@
> #define GRU_OPERATION_TIMEOUT	(((cycles_t) local_cpu_data->itc_freq)*10)
> #define CLKS2NSEC(c)		((c) *1000000000 / local_cpu_data->itc_freq)
> #else
> -#include <linux/sync_core.h>
> +#include <asm/sync_core.h>
> #include <asm/tsc.h>
> #define GRU_OPERATION_TIMEOUT	((cycles_t) tsc_khz*10*1000)
> #define CLKS2NSEC(c)		((c) * 1000000 / tsc_khz)
> diff --git a/drivers/misc/sgi-gru/grukservices.c
> b/drivers/misc/sgi-gru/grukservices.c
> index 0ea923fe6371..ce03ff3f7c3a 100644
> --- a/drivers/misc/sgi-gru/grukservices.c
> +++ b/drivers/misc/sgi-gru/grukservices.c
> @@ -16,10 +16,10 @@
> #include <linux/miscdevice.h>
> #include <linux/proc_fs.h>
> #include <linux/interrupt.h>
> -#include <linux/sync_core.h>
> #include <linux/uaccess.h>
> #include <linux/delay.h>
> #include <linux/export.h>
> +#include <asm/sync_core.h>
> #include <asm/io_apic.h>
> #include "gru.h"
> #include "grulib.h"
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index e8919995d8dd..e107f292fc42 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -7,7 +7,6 @@
> #include <linux/sched.h>
> #include <linux/mm_types.h>
> #include <linux/gfp.h>
> -#include <linux/sync_core.h>
> 
> /*
>  * Routines for handling mm_structs
> diff --git a/include/linux/sync_core.h b/include/linux/sync_core.h
> deleted file mode 100644
> index 013da4b8b327..000000000000
> --- a/include/linux/sync_core.h
> +++ /dev/null
> @@ -1,21 +0,0 @@
> -/* SPDX-License-Identifier: GPL-2.0 */
> -#ifndef _LINUX_SYNC_CORE_H
> -#define _LINUX_SYNC_CORE_H
> -
> -#ifdef CONFIG_ARCH_HAS_SYNC_CORE_BEFORE_USERMODE
> -#include <asm/sync_core.h>
> -#else
> -/*
> - * This is a dummy sync_core_before_usermode() implementation that can be used
> - * on all architectures which return to user-space through core serializing
> - * instructions.
> - * If your architecture returns to user-space through non-core-serializing
> - * instructions, you need to write your own functions.
> - */
> -static inline void sync_core_before_usermode(void)
> -{
> -}
> -#endif
> -
> -#endif /* _LINUX_SYNC_CORE_H */
> -

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com