linux-kernel - Re: [RESEND PATCH v2 1/3] x86/fred: Allow variable-sized event frame

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Message-ID: <CAMzpN2jRe1dOpKsAxCOG3RVS_x9yq-1phYuzm9ObdpJYQjb++Q@mail.gmail.com>
Date: Wed, 19 Mar 2025 09:55:51 -0400
From: Brian Gerst <brgerst@...il.com>
To: Xin Li <xin@...or.com>
Cc: linux-kernel@...r.kernel.org, luto@...nel.org, tglx@...utronix.de, 
	mingo@...nel.org, bp@...en8.de, dave.hansen@...ux.intel.com, x86@...nel.org, 
	hpa@...or.com, peterz@...radead.org
Subject: Re: [RESEND PATCH v2 1/3] x86/fred: Allow variable-sized event frame

On Wed, Mar 19, 2025 at 2:47 AM Xin Li <xin@...or.com> wrote:
>
> On 3/18/2025 6:53 AM, Brian Gerst wrote:
> >> -#define INIT_THREAD_INFO(tsk)                  \
> >> -{                                              \
> >> -       .flags          = 0,                    \
> >> +#define INIT_THREAD_INFO(tsk)                                          \
> >> +{                                                                      \
> >> +       .flags          = 0,                                            \
> >> +       .user_pt_regs   = (struct pt_regs *)TOP_OF_INIT_STACK - 1,      \
> >
> > Use __top_init_kernel_stack here.
>
> Will do.
>
> >
> >>   }
> >>
> >>   #else /* !__ASSEMBLER__ */
> >> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> >> index 91f6ff618852..58c1cd4ca60a 100644
> >> --- a/arch/x86/kernel/process.c
> >> +++ b/arch/x86/kernel/process.c
> >> @@ -108,6 +108,28 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
> >>          return 0;
> >>   }
> >>
> >> +/*
> >> + * Initialize thread_info.user_pt_regs for IDT event delivery.
> >> + *
> >> + * For IDT user level event delivery, a pt_regs structure is pushed by both
> >> + * hardware and software and always resides at a fixed offset from top of
> >> + * current task kernel stack, thus thread_info.user_pt_regs is a per-task
> >> + * constant and NEVER changes after initialization.
> >> + *
> >> + * While for FRED user level event delivery, user_pt_regs is updated in
> >> + * fred_entry_from_user() immediately after user level event delivery.
> >> + *
> >> + * Note: thread_info.user_pt_regs of the init task is initialized at build
> >> + * time.
> >> + */
> >> +void arch_init_user_pt_regs(struct task_struct *tsk)
> >> +{
> >> +       unsigned long top_of_stack = (unsigned long)task_stack_page(tsk) + THREAD_SIZE;
> >> +
> >> +       top_of_stack -= TOP_OF_KERNEL_STACK_PADDING;
> >> +       tsk->thread_info.user_pt_regs = (struct pt_regs *)top_of_stack - 1;
> >> +}
> >
> > Can this be put into arch_dup_task_struct() instead of creating another hook?
>
> I wanted to do it at the beginning but task stack is no longer part of
> the task_struct on x86.  Make sense?

I see that now.  My concern here is that using a weak function makes
all other arches pay the cost of an empty function call.  That's why
many hooks are static linlines or macros, especially if the default is
a no-op.


Brian Gerst