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SubjectRe: [PATCH v6 11/10] x86/retpoline: Avoid return buffer underflows on context switch
On Mon, Jan 08, 2018 at 03:56:30PM -0800, Linus Torvalds wrote:
> On Mon, Jan 8, 2018 at 3:44 PM, David Woodhouse <dwmw2@infradead.org> wrote:
> >
> > To guard against this fill the return buffer with controlled
> > content during context switch. This prevents any underflows.
>
> Ugh. I really dislike this patch. Everything else in the retpoline
> patches makes me go "ok, that's reasonable". This one makes me go
> "Eww".
>
> It's hacky, it's ugly, and it looks pretty expensive too.

Modern cores are quite fast at executing calls.

>
> Is there really nothing more clever we can do?

We could be a cleverer in selecting how many dummy calls to do.
But that would likely be fragile and hard to maintain
and likely be more complicated, and I doubt it would buy that much.

Don't really have a better proposal, sorry.

-Andi

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