Re: [Xen-devel] Q:pt_base in COMPAT mode offset by two pages. Was:Re: [PATCH 02/11] xen/x86: Use memblock_reserve for sensitive areas.

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 21.08.12 at 21:03, Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx> wrote:
> On Tue, Aug 21, 2012 at 01:27:32PM -0400, Konrad Rzeszutek Wilk wrote:
>> On Mon, Aug 20, 2012 at 10:13:05AM -0400, Konrad Rzeszutek Wilk wrote:
>> > On Fri, Aug 17, 2012 at 06:35:12PM +0100, Stefano Stabellini wrote:
>> > > On Thu, 16 Aug 2012, Konrad Rzeszutek Wilk wrote:
>> > > > instead of a big memblock_reserve. This way we can be more
>> > > > selective in freeing regions (and it also makes it easier
>> > > > to understand where is what).
>> > > > 
>> > > > [v1: Move the auto_translate_physmap to proper line]
>> > > > [v2: Per Stefano suggestion add more comments]
>> > > > Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
>> > > 
>> > > much better now!
>> > 
>> > Thought interestingly enough it breaks 32-bit dom0s (and only dom0s).
>> > Will have a revised patch posted shortly.
>> 
>> 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?

Actually, the adjustment turns out to be correct: The page
tables for a 32-on-64 dom0 get allocated in the order "first L1",
"first L2", "first L3", so the offset to the page table base is
indeed 2. When reading xen/include/public/xen.h's comment
very strictly, this is not a violation (since there nothing is said
that the first thing in the page table space is pointed to by
pt_base; I admit that this seems to be implied though, namely
do I think that it is implied that the page table space is the
range [pt_base, pt_base + nt_pt_frames), whereas that
range here indeed is [pt_base - 2, pt_base - 2 + nt_pt_frames),
which - without a priori knowledge - the kernel would have
difficulty to figure out).

Below is a debugging patch I used to see the full picture, if you
want to double check.

One thing I also noticed is that nr_pt_frames apparently is
one too high in this case, as the L4 is not really part of the
page tables from the kernel's perspective (and not represented
anywhere in the corresponding VA range).

Jan

--- a/xen/arch/x86/domain_build.c
+++ b/xen/arch/x86/domain_build.c
@@ -940,6 +940,7 @@ int __init construct_dom0(
     si->flags       |= (xen_processor_pmbits << 8) & SIF_PM_MASK;
     si->pt_base      = vpt_start + 2 * PAGE_SIZE * !!is_pv_32on64_domain(d);
     si->nr_pt_frames = nr_pt_pages;
+printk("PT#%lx\n", si->nr_pt_frames);//temp
     si->mfn_list     = vphysmap_start;
     snprintf(si->magic, sizeof(si->magic), "xen-3.0-x86_%d%s",
              elf_64bit(&elf) ? 64 : 32, parms.pae ? "p" : "");
@@ -1115,6 +1116,10 @@ int __init construct_dom0(
                 process_pending_softirqs();
         }
     }
+show_page_walk(vpt_start);//temp
+show_page_walk(si->pt_base);//temp
+show_page_walk(v_start);//temp
+show_page_walk(v_end - 1);//temp
 
     if ( initrd_len != 0 )
     {


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