Re: [PATCH] x86: Construct 32 bit boot time page tables in nativeformat.

[H. Peter Anvin]

Jeremy Fitzhardinge wrote:
> H. Peter Anvin wrote:
> > No, if Xen wasn't an issue there wouldn't be anything to do for the 
> > PAE case at all (since the PGD is trivial.)
> >
> > Copying PMDs is more or less an analogous case of the !PAE case, once 
> > the allocation is already done.  The allocation should be trivial 
> > though, since this would be a one-time thing.
>
> I think we're in vehement agreement here.  In either case, its just a 
> matter of something like:
>
>    memcpy(pgd, &pgd[USER_PTRS_PER_PGD], sizeof(pgd_t) * 
> KERNEL_PTRS_PER_PGD);
>     
> which would work for both PAE and non-PAE.
>
> > > It would be easy enough to add some code on xen side to look for pmd 
> > > aliases when using/pinning a pagetable, and allocate'n'copy a new pmd 
> > > page as needed.  That way the core code can ignore the issue.
> >
> > As much as I'd rather see Xen fixing this than having it continue to 
> > impact the kernel, I presume it will take some time to flush the 
> > broken hypervisors out? 
>
> Sorry, I was unclear.  I meant in the purely Xen-specific parts of the 
> kernel (arch/x86/xen).  It wouldn't require a hypervisor change.

Oh, that makes that option much more viable and probably preferrable.

	-hpa
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