Subject: Re: [PATCH v17 09/47] arm64, dept: add support CONFIG_ARCH_HAS_DEPT_SUPPORT to arm64

From: Byungchul Park <[email-masked]>
Date: Mon, 13 Oct 2025 13:28:07 +0900

On Fri, Oct 03, 2025 at 03:36:42PM +0100, Mark Rutland wrote:
> On Thu, Oct 02, 2025 at 05:12:09PM +0900, Byungchul Park wrote:
> > dept needs to notice every entrance from user to kernel mode to treat
> > every kernel context independently when tracking wait-event dependencies.
> > Roughly, system call and user oriented fault are the cases.
> >
> > Make dept aware of the entrances of arm64 and add support
> > CONFIG_ARCH_HAS_DEPT_SUPPORT to arm64.
> >
> > Signed-off-by: Byungchul Park <[email-masked]>
> > ---
> >  arch/arm64/Kconfig          | 1 +
> >  arch/arm64/kernel/syscall.c | 7 +++++++
> >  arch/arm64/mm/fault.c       | 7 +++++++
> >  3 files changed, 15 insertions(+)
> >
> > diff --git a/arch/arm64/Kconfig b/arch/arm64/Kconfig
> > index e9bbfacc35a6..a8fab2c052dc 100644
> > --- a/arch/arm64/Kconfig
> > +++ b/arch/arm64/Kconfig
> > @@ -281,6 +281,7 @@ config ARM64
> >       select USER_STACKTRACE_SUPPORT
> >       select VDSO_GETRANDOM
> >       select VMAP_STACK
> > +     select ARCH_HAS_DEPT_SUPPORT
> >       help
> >         ARM 64-bit (AArch64) Linux support.
> >
> > diff --git a/arch/arm64/kernel/syscall.c b/arch/arm64/kernel/syscall.c
> > index c442fcec6b9e..bbd306335179 100644
> > --- a/arch/arm64/kernel/syscall.c
> > +++ b/arch/arm64/kernel/syscall.c
> > @@ -7,6 +7,7 @@
> >  #include <linux/ptrace.h>
> >  #include <linux/randomize_kstack.h>
> >  #include <linux/syscalls.h>
> > +#include <linux/dept.h>
> >
> >  #include <asm/debug-monitors.h>
> >  #include <asm/exception.h>
> > @@ -96,6 +97,12 @@ static void el0_svc_common(struct pt_regs *regs, int scno, int sc_nr,
> >        * (Similarly for HVC and SMC elsewhere.)
> >        */
> >
> > +     /*
> > +      * This is a system call from user mode.  Make dept work with a
> > +      * new kernel mode context.
> > +      */
> > +     dept_update_cxt();
> 
> As Mark Brown pointed out in his replies, this patch is missing a whole
> bunch of cases and does not work correctly as-is.
> 
> As Dave Hansen pointed out on the x86 patch, you shouldn't do this
> piecemeal in architecture code, and should instead work with the
> existing context tracking, e.g. by adding logic to
> enter_from_user_mode() and exit_to_user_mode(), or by reusing some

I will consider it.  However, I need to check if there are not any waits
and events before enter_from_user_mode(), or after exit_to_user_mode()
since those functions aren't the outmost functions for kernel mode C
code anyway.

	Byungchul

> existing context tracking logic that's called there.
> 
> Mark.