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SubjectRe: [RFC PATCH 2/5] entry: Add calls for save/restore auxiliary pt_regs
On Tue, Aug 09, 2022 at 02:05:15PM +0200, Borislav Petkov wrote:
> On Fri, Aug 05, 2022 at 10:30:06AM -0700, ira.weiny@intel.com wrote:
> > From: Ira Weiny <ira.weiny@intel.com>
> >
> > Some architectures have auxiliary pt_regs space available to store
> > information on the stack during exceptions. This information is easier
> > to obtain and store within C code rather than in arch specific assembly.
>
> There are others?

Other archs? not now.

>
> Because I would've done this whole thing in arch/x86/ only...

Thomas did a lot of work to make the entry code generic. So I was keeping that
work consistent. This also helps to ensure I did not miss any places.

>
> > diff --git a/kernel/entry/common.c b/kernel/entry/common.c
> > index 8c0f334c4b75..a70a0f314aee 100644
> > --- a/kernel/entry/common.c
> > +++ b/kernel/entry/common.c
> > @@ -317,7 +317,7 @@ noinstr irqentry_state_t irqentry_enter(struct pt_regs *regs)
> >
> > if (user_mode(regs)) {
> > irqentry_enter_from_user_mode(regs);
> > - return ret;
> > + goto aux_save;
>
> Why do you have to goto and do the instrumentation sandwitch around it
> at the goto label?
>
> Why not simply do
>
> if (user_mode(regs)) {
> irqentry_enter_from_user_mode(regs);
> arch_save_aux_pt_regs(regs);
> return ret;

I don't believe this is correct because instrumentation is not enabled here.
See below.[1]

There are 3 exit paths from irqentry_enter(). All must call
arch_save_aux_pt_regs(). I felt the maintenance of the code would be easier if
I did not scatter those calls through the function and simply exited 1
place.[1]

FWICT calling instrumentation_{begin,end}() is a noop in production code. So
there is no real cost to calling instrumentation_begin() -> end -> begin ->
end. And the goto seemed low enough overhead. Given the current discussion I
admit I may have made the wrong choice. But I think I would add some comments
to the below to help future developers.

Ira

[1]

--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -317,6 +317,9 @@ noinstr irqentry_state_t irqentry_enter(struct pt_regs *regs)

if (user_mode(regs)) {
irqentry_enter_from_user_mode(regs);
+ instrumentation_begin();
+ arch_save_aux_pt_regs(regs);
+ instrumentation_end();
return ret;
}

@@ -353,6 +356,7 @@ noinstr irqentry_state_t irqentry_enter(struct pt_regs *regs)
ct_irq_enter();
instrumentation_begin();
trace_hardirqs_off_finish();
+ arch_save_aux_pt_regs(regs);
instrumentation_end();

ret.exit_rcu = true;
@@ -369,6 +373,7 @@ noinstr irqentry_state_t irqentry_enter(struct pt_regs *regs)
instrumentation_begin();
rcu_irq_enter_check_tick();
trace_hardirqs_off_finish();
+ arch_save_aux_pt_regs(regs);
instrumentation_end();

return ret;

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