Subject: Re: [PATCH 0/6] Use virtual page class key protection mechanism for speeding up guest page fault

From: Benjamin Herrenschmidt <[email-masked]>
Date: Sun, 29 Jun 2014 21:26:06 +1000

On Sun, 2014-06-29 at 16:47 +0530, Aneesh Kumar K.V wrote:

> To achieve the above we use virtual page calss protection mechanism for
> covering (2) and (3). For both the above case we mark the hpte
> valid, but associate the page with virtual page class index 30 and 31.
> The authority mask register is configured such that class index 30 and 31
> will have read/write denied. The above change results in a key fault
> for (2) and (3). This allows us to forward a NO_HPTE fault directly to guest
> without doing the expensive hash pagetable lookup.

So we have a measurable performance benefit (about half a second out of
8) but you didn't explain the drawback here which is to essentially make
it impossible for guests to exploit virtual page class keys, or did you
find a way to still make that possible ?

As it-is, it's not a huge issue for Linux but we might have to care with
other OSes that do care...

Do we have a way in PAPR to signify to the guest that the keys are not
available ?

Cheers,
Ben.