Re: [PATCH v17 08/47] x86_64, dept: add support CONFIG_ARCH_HAS_DEPT_SUPPORT to x86_64

Byungchul Park
2025-10-03T01:12:08Z

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