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Re: [Xen-devel] [PATCH v8 1/9] x86/xpti: avoid copying L4 page table contents when possible

At 19:11 +0200 on 21 Apr (1524337893), Juergen Gross wrote:
> On 21/04/18 15:32, Tim Deegan wrote:
> > At 09:44 +0200 on 19 Apr (1524131080), Juergen Gross wrote:
> >> Another alternative would be to pass another flag to the callers to
> >> signal the need for a flush. This would require quite some modifications
> >> to shadow code I'd like to avoid, though. OTOH this way we could combine
> >> flushing the tlb and the root page tables. Tim, any preferences?
> > 
> > This sounds a promising direction but it should be doabl without major
> > surgery to the shadow code.  The shadow code already leaves old sl4es
> > visible (in TLBs) when it's safe to do so, so I think the right place
> > to hook this is on the receiving side of the TLB flush IPI.  IOW as
> > long as:
> >  - you copy the L4 on context switch; and
> >  - you copy it on the TLB flush IPI is received
> > then you can rely on the existing TLB flush mechanisms to do what you need.
> > And shadow doesn't have to behave differently from 'normal' PV MM.
> It is not so easy. The problem is that e.g. a page fault will flush the
> TLB entry for the page in question, but it won't lead to the L4 to be
> copied.

Oh yes, I see; thanks for the explanation.  It might be worth copying
what the hardware does here, and checking/propagating the relevant l4e
in the PV pagefault handler.


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