Re: [PATCH v6 09/12] x86/process: Clear PASID state for a newly forked/cloned thread

[Andy Lutomirski]

On Mon, Jul 13, 2020 at 4:48 PM Fenghua Yu <fenghua.yu@xxxxxxxxx> wrote:
>
> The PASID state has to be cleared on forks, since the child has a
> different address space. The PASID is also cleared for thread clone. While
> it would be correct to inherit the PASID in this case, it is unknown
> whether the new task will use ENQCMD. Giving it the PASID "just in case"
> would have the downside of increased context switch overhead to setting
> the PASID MSR.
>
> Since #GP faults have to be handled on any threads that were created before
> the PASID was assigned to the mm of the process, newly created threads
> might as well be treated in a consistent way.

Just how much context switch overhead are we talking about here?
Unless the CPU works differently than I expect, I would guess you are
saving exactly zero cycles.  What am I missing?

--Andy


>
> Suggested-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Fenghua Yu <fenghua.yu@xxxxxxxxx>
> Reviewed-by: Tony Luck <tony.luck@xxxxxxxxx>
> ---
> v2:
> - Modify init_task_pasid().
>
>  arch/x86/kernel/process.c | 18 ++++++++++++++++++
>  1 file changed, 18 insertions(+)
>
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index f362ce0d5ac0..1b1492e337a6 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -121,6 +121,21 @@ static int set_new_tls(struct task_struct *p, unsigned long tls)
>                 return do_set_thread_area_64(p, ARCH_SET_FS, tls);
>  }
>
> +/* Initialize the PASID state for the forked/cloned thread. */
> +static void init_task_pasid(struct task_struct *task)
> +{
> +       struct ia32_pasid_state *ppasid;
> +
> +       /*
> +        * Initialize the PASID state so that the PASID MSR will be
> +        * initialized to its initial state (0) by XRSTORS when the task is
> +        * scheduled for the first time.
> +        */
> +       ppasid = get_xsave_addr(&task->thread.fpu.state.xsave, XFEATURE_PASID);
> +       if (ppasid)
> +               ppasid->pasid = INIT_PASID;
> +}
> +
>  int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
>                     unsigned long arg, struct task_struct *p, unsigned long tls)
>  {
> @@ -174,6 +189,9 @@ int copy_thread_tls(unsigned long clone_flags, unsigned long sp,
>         task_user_gs(p) = get_user_gs(current_pt_regs());
>  #endif
>
> +       if (static_cpu_has(X86_FEATURE_ENQCMD))
> +               init_task_pasid(p);
> +
>         /* Set a new TLS for the child thread? */
>         if (clone_flags & CLONE_SETTLS)
>                 ret = set_new_tls(p, tls);
> --
> 2.19.1
>