Message-ID: <20251003011201.GD75385@system.software.com>
Date: Fri, 3 Oct 2025 10:12:01 +0900
From: Byungchul Park <byungchul@...com>
To: Dave Hansen <dave.hansen@...el.com>
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Subject: Re: [PATCH v17 08/47] x86_64, dept: add support
 CONFIG_ARCH_HAS_DEPT_SUPPORT to x86_64

On Thu, Oct 02, 2025 at 08:22:29AM -0700, Dave Hansen wrote:
> On 10/2/25 01:12, 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.
> 
> "Roughly"?

I will change it to a better one.

> >  #define __SYSCALL(nr, sym) extern long __x64_##sym(const struct pt_regs *);
> >  #define __SYSCALL_NORETURN(nr, sym) extern long __noreturn __x64_##sym(const struct pt_regs *);
> > @@ -86,6 +87,12 @@ static __always_inline bool do_syscall_x32(struct pt_regs *regs, int nr)
> >  /* Returns true to return using SYSRET, or false to use IRET */
> >  __visible noinstr bool do_syscall_64(struct pt_regs *regs, int nr)
> >  {
> > +     /*
> > +      * This is a system call from user mode.  Make dept work with a
> > +      * new kernel mode context.
> > +      */
> > +     dept_update_cxt();
> > +
> >       add_random_kstack_offset();
> >       nr = syscall_enter_from_user_mode(regs, nr);
> 
> Please take a look in syscall_enter_from_user_mode(). You'll see the
> quite nicely-named function: enter_from_user_mode(). That might be a
> nice place to put code that you want to run when the kernel is entered
> from user mode.

I wanted to put dept_update_cxt() to the very beginning of c code but..
yeah enter_from_user_mode() looks fine or even better.  Thanks a lot.

> > diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> > index 998bd807fc7b..017edb75f0a0 100644
> > --- a/arch/x86/mm/fault.c
> > +++ b/arch/x86/mm/fault.c
> > @@ -19,6 +19,7 @@
> >  #include <linux/mm_types.h>
> >  #include <linux/mm.h>                        /* find_and_lock_vma() */
> >  #include <linux/vmalloc.h>
> > +#include <linux/dept.h>
> >
> >  #include <asm/cpufeature.h>          /* boot_cpu_has, ...            */
> >  #include <asm/traps.h>                       /* dotraplinkage, ...           */
> > @@ -1219,6 +1220,12 @@ void do_user_addr_fault(struct pt_regs *regs,
> >       tsk = current;
> >       mm = tsk->mm;
> >
> > +     /*
> > +      * This fault comes from user mode.  Make dept work with a new
> > +      * kernel mode context.
> > +      */
> > +     dept_update_cxt();
> No, this fault does not come from user mode. That's why we call it "user
> addr" fault, not "user mode" fault. You end up here if, for instance,
> the kernel faults doing a copy_from_user().

My bad.  Thank you.  I will fix it.  Thank you very much.

	Byungchul

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