Re: [Xen-devel] [RFC PATCH 4/4] arm/mem_access: Add software guest-page-table walk

[Julien Grall <julien.grall@xxxxxxx>]

On 05/09/2017 08:17 AM, Sergej Proskurin wrote:
> Hi,
>
> > > What you currently do is try gva_to_ipa and if it does not work >> you will 
> > > call p2m_gva_to_ipa. This sounds a bit pointless to me and
> > > waste of time if the underlying memory of stage-1 page table is >>
> protected. > > But we don't know that the stage-1 page table is
> protected until the > hardware based lookup fails. I can turn your
> argument around and say > doing the software based lookup is pointless
> and a waste of time > when the stage-1 table is not protected. Which is
> by the way what I > would expect to see in most cases.
>
> I agree with Tamas: I also believe that in most cases the stage-1
> translation table won't be protected. So, in my opinion, falling back to
> software (which will be presumablly a rare operation) is the better
> approach.
Well, you both consider that it is better to do the fallback by assuming 
the TLBs will always cache the intermediate translations (e.g VA -> IPA) 
and not only the full S1 -> S2 translation (e.g VA -> PA).
Cheers,

--
Julien Grall

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