Re: [PATCH 06/10] KVM: arm/arm64: Only clean the dcache on translation fault

Christoffer Dall
2017-10-17T14:36:34Z

On Tue, Oct 17, 2017 at 10:34:15AM +0100, Marc Zyngier wrote:
> On 16/10/17 21:08, Christoffer Dall wrote:
> > On Mon, Oct 09, 2017 at 04:20:28PM +0100, Marc Zyngier wrote:
> >> The only case where we actually need to perform a dache maintenance
> >> is when we map the page for the first time, and subsequent permission
> >> faults do not require cache maintenance. Let's make it conditional
> >> on not being a permission fault (and thus a translation fault).
> >
> > Why do we actually need to do any dcache maintenance when faulting in a
> > page?
> >
> > Is this for the case when the stage 1 MMU is disabled, or to support
> > guest mappings using uncached attributes?
>
> These are indeed the two cases that require cleaning the dcache to PoC.
>
> > Can we do better, for example
> > by only flushing the cache if the guest MMU is disabled?
>
> The guest MMU being disabled is easy. But the uncached mapping is much
> trickier, and would involve parsing the guest page tables. Not something
> I'm really eager to implement.
>

Hmm, if the guest actually maps memory uncached, wouldn't it have to
invalidate caches itself, or is this the annoying thing where disabling
the MMU on hardware that doesn't have stage 2 would in fact always
completely bypass the cache, and therefore we have to do this work?

Sorry, I have forgotten all the details here, but wanted to make sure
we're not being overly careful.

Thanks,
-Christoffer