Re: [Xen-devel] [PATCH 01/10] VMX: Enable EPT A/D bit support

[Tim Deegan <tim@xxxxxxx>]

At 17:26 +0800 on 10 Apr (1428686789), Kai Huang wrote:
> 
> 
> On 04/10/2015 04:54 PM, Tim Deegan wrote:
> > At 14:40 +0800 on 10 Apr (1428676858), Kai Huang wrote:
> >>> In fact, thinking about that, we should probably be setting _all_ the
> >>> A bits to 1, even in leaf entries, to avoid the overhead of the MMU
> >>> having to write them later.
> >> I suppose you mean A bits of leaf entries of new shattered 4K pages? Yes
> >> we can, but looks it's more reasonable to inherit according to original
> >> A bit of super page.
> > Yes, inheriting is best, but we should have already set the A bit of the
> > superpage to 1 as well, for the same reasons.  IOW we should set every A
> > bit in the whole system to 1 all the time.
> >
> > BTW, this is what we do with A/D bits in normal pagetables in Xen too
> > -- the default is to set them all the time, so the MMU doesn't have to
> > fix them up later.
> Sure then we can set all A bit to 1, which can be done by setting A bit 
> properly in ept_p2m_type_to_flags I think.
> 
> What does normal pagetables mean here? I don't think you mean shadow 
> page table, as shadow page table needs to sync A/D bits with guest 
> table, right?

I meant xen's own pagetables, that it maps its own text and data with.
But IIRC we also set the Accessed bit in all shadow pagetable entries
-- fixing up the A/D bits in the guest PTEs happens in the pagefault
handler.

Tim.

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