linux-kernel - Re: Q:pt_base in COMPAT mode offset by two pages. Was:Re: [Xen-devel] [PATCH 02/11] xen/x86: Use memblock

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Message-Id: <503504FE0200007800096F08@nat28.tlf.novell.com>
Date:	Wed, 22 Aug 2012 15:12:46 +0100
From:	"Jan Beulich" <JBeulich@...e.com>
To:	"Konrad Rzeszutek Wilk" <konrad.wilk@...cle.com>
Cc:	"Stefano Stabellini" <stefano.stabellini@...citrix.com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Q:pt_base in COMPAT mode offset by two pages. Was:Re:
 [Xen-devel] [PATCH 02/11] xen/x86: Use memblock_reserve for sensitive
 areas.

>>> On 21.08.12 at 21:03, Konrad Rzeszutek Wilk <konrad.wilk@...cle.com> wrote:
> On Tue, Aug 21, 2012 at 01:27:32PM -0400, Konrad Rzeszutek Wilk wrote:
>> Jan, I thought something odd. Part of this code replaces this:
>> 
>> 	memblock_reserve(__pa(xen_start_info->mfn_list),
>> 		xen_start_info->pt_base - xen_start_info->mfn_list);
>> 
>> with a more region-by-region area. What I found out that if I boot this
>> as 32-bit guest with a 64-bit hypervisor the xen_start_info->pt_base is
>> actually wrong.
>> 
>> Specifically this is what bootup says:
>> 
>> (good working case - 32bit hypervisor with 32-bit dom0):
>> (XEN)  Loaded kernel: c1000000->c1a23000
>> (XEN)  Init. ramdisk: c1a23000->cf730e00
>> (XEN)  Phys-Mach map: cf731000->cf831000
>> (XEN)  Start info:    cf831000->cf83147c
>> (XEN)  Page tables:   cf832000->cf8b5000
>> (XEN)  Boot stack:    cf8b5000->cf8b6000
>> (XEN)  TOTAL:         c0000000->cfc00000
>> 
>> [    0.000000] PT: cf832000 (f832000)
>> [    0.000000] Reserving PT: f832000->f8b5000
>> 
>> And with a 64-bit hypervisor:
>> 
>> XEN) VIRTUAL MEMORY ARRANGEMENT:
>> (XEN)  Loaded kernel: 00000000c1000000->00000000c1a23000
>> (XEN)  Init. ramdisk: 00000000c1a23000->00000000cf730e00
>> (XEN)  Phys-Mach map: 00000000cf731000->00000000cf831000
>> (XEN)  Start info:    00000000cf831000->00000000cf8314b4
>> (XEN)  Page tables:   00000000cf832000->00000000cf8b6000
>> (XEN)  Boot stack:    00000000cf8b6000->00000000cf8b7000
>> (XEN)  TOTAL:         00000000c0000000->00000000cfc00000
>> (XEN)  ENTRY ADDRESS: 00000000c16bb22c
>> 
>> [    0.000000] PT: cf834000 (f834000)
>> [    0.000000] Reserving PT: f834000->f8b8000
>> 
>> So the pt_base is offset by two pages. And looking at c/s 13257
>> its not clear to me why this two page offset was added?

Honestly, without looking through this in greater detail I don't
recall. That'll have to wait possibly until after the summit, though.
I can't exclude that this is just a forgotten leftover from an earlier
version of the patch. I would have thought this was to account
for the L4 tables that the guest doesn't see, but
- this should only be a single page
- this should then also (or rather instead) be subtracted from
  nr_pt_frames
so that's likely not it.

>> The toolstack works fine - so launching 32-bit guests either
>> under a 32-bit hypervisor or 64-bit works fine:
>> ] domainbuilder: detail: xc_dom_alloc_segment:   page tables  : 0xcf805000 -> 
> 0xcf885000  (pfn 0xf805 + 0x80 pages)
>> [    0.000000] PT: cf805000 (f805000)
>> 
> 
> And this patch on top of the others fixes this..

I didn't look at this in too close detail, but I started to get
afraid that you might be making the code dependent on
many hypervisor implementation details. And should the
above turn out to be a bug in the hypervisor, I hope your
kernel side changes won't make it impossible to fix that bug.

Jan

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