Re: x86, mce, Use user return notifier in mce

[Huang Ying <ying.huang@intel.com>]

On Thu, 2012-01-12 at 17:46 -0800, Tony Luck wrote:
> On Thu, Jan 12, 2012 at 4:36 PM, Huang Ying <ying.huang@intel.com> wrote:
> > Replace the home-made TIF_MCE_NOTIFY based code in MCE with user
> > return notifier.
> 
> When I looked at this before, I found that the existing user return notifier had
> the meaning "call a function before THIS CPU returns to user space". Use in KVM
> was to update some MSR that needed adjustment before a cpu ran another user
> process.
> 
> The MCE code wanted something slightly different: "call a function before THIS
> PROCESS returns to user space". So my prototype code from last year made a
> whole new set of interfaces - similar in style to the user return
> notifier, but with
> the MCE semantics.
> 
> At first glance it looks like you are just using the user return
> notifier code (perhaps
> I'm mis-reading the diff?). This won't work - it's possible for a
> context switch, and
> then the process that hit the MCE may get moved to another cpu, when it will
> be run. Meanwhile we'll execute our function in the context of some
> other process.

You are right.  User return notifier can not be used for SRAR.  I think
that may be useful for SRAO.  Where we need a way to do notify earlier
in case of the corresponding work_queue item is not executed in time.

Best Regards,
Huang Ying