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SubjectRe: [PATCH 10/39] x86/entry/32: Handle Entry from Kernel-Mode on Entry-Stack
On Fri, Jul 13, 2018 at 11:26:54PM -0700, Andy Lutomirski wrote:
> > So based on that, I did the above because the entry-stack is a per-cpu
> > data structure and I am not sure that we always return from the exception
> > on the same CPU where we got it. Therefore the path is called
> > PARANOID_... :)
>
> But we should just be able to IRET and end up right back on the entry
> stack where we were when we got interrupted.

Yeah, but using another CPUs entry-stack is a bad idea, no? Especially
since the owning CPU might have overwritten our content there already.

> On x86_64, we *definitely* can’t schedule in NMI, MCE, or #DB because
> we’re on a percpu stack. Are you *sure* we need this patch?

I am sure we need this patch, but not 100% sure that we really can
change CPUs in this path. We are not only talking about NMI, #MC and
#DB, but also about #GP and every other exception that can happen while
writing segments registers or on iret. With this implementation we are
on the safe side for this unlikely slow-path.

Regards,

Joerg
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