linux-kernel - Re: [PATCH] x86: Construct 32 bit boot time page tables in native format.

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Message-ID: <47992DD9.8060604@goop.org>
Date:	Thu, 24 Jan 2008 16:31:21 -0800
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Ian Campbell <ijc@...lion.org.uk>, Ingo Molnar <mingo@...e.hu>,
	Mika Penttilä <mika.penttila@...umbus.fi>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH] x86: Construct 32 bit boot time page tables in native
 format.

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.

    J
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