Re: [Xen-devel] [PATCH] xen: provide pse36 cpuid bit

[Tim Deegan <tim@xxxxxxx>]

At 15:53 +0100 on 28 Oct (1319817196), Tim Deegan wrote:
> At 16:20 +0200 on 28 Oct (1319818819), Christoph Egger wrote:
> > On 10/27/11 17:30, Keir Fraser wrote:
> > >On 27/10/2011 16:15, "Tim Deegan"<tim@xxxxxxx>  wrote:
> > >
> > >>If you mean this:
> > >>
> > >>  * PSE disabled / PSE36
> > >>  * We don't support any modes other than PSE enabled, PSE36 disabled.
> > >>  * Neither of those would be hard to change, but we'd need to be able to
> > >>  * deal with shadows made in one mode and used in another.
> > >>
> > >>the worry was that we'd need a whole nother shadow mode to handle the
> > >>case where one VCPU was in normal 32-bit and another was in PSE36 (since
> > >>they can't share shadows).
> > >>
> > >>As it happens the current code does detect PSE-disabled in shadow mode
> > >>but just DTRT for the current VCPU, so a mix of PSE-enabled and
> > >>PSE-disabled VCPUs will get unpredicatble results from shadow
> > >>pagetables. :(
> > >>
> > >>Which means that supporting PSE36 to the same degree (i.e. assuming all
> > >>VCPUs behave the same, or if they don't they don't share pagetables)
> > >>would be OK too. :)
> > >
> > >Ah, I see. Yes, I guessed it would be supported to just the same degree as
> > >'basic' PSE. The likelihood of pagetables being shared across different
> > >pagetable-related CR4 settings? Not great, we hope. :-)
> > 
> > Is the patch acceptable as it is ? 
> 
> I'll put together a patch with basic PSE36 support next week.  I don't
> think we're in a terrible hurry to check in the CPUID change before then.

Supporting PSE36 turned out to be trickier than I had thought - the
trick in the guest-pagetable walker of making up an l1e with the
equivalent semantics to the PSE l2e doesn't work for PSE36 because there
isn't space for the extra address bits in a 32bit l1e. :(  I didn't fancy
changing that interface right now because it would need a fair bit of
fixing up in the shadow code, but I might come back to it when I have
more time. 

In the meantime I've applied Christoph's patch.

Cheers,

Tim.

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