Re: [PATCH 5/6] docs: Document start_info changes in Xen 4.2.

[Keir Fraser <keir@xen.org>]

On 18/03/2013 10:55, "Jan Beulich" <JBeulich@suse.com> wrote:

>>>> On 15.03.13 at 20:51, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com> wrote:
>> The  25833:bb85bbccb1c9. "x86/32-on-64: adjust Dom0 initial page table
>> layout"
>> fixes a bug in the reported value of pt_base versus where the page tables
>> actually start. This documents this in the start of the world header note.
>> 
>> This clarifies the implied understanding that the page table space is
>> pointed by pt_base. As in it is ".. implied that the range of page-tables
>> is the range [pt_base, pt_base + nr_pt_frames), whereas that that range
>> here indeed is [pt_base - 2, pt_base -2 + nr_pt_frames)" (Jan Beulich).
>> 
>> Also make it crystal clear that pt_base == %cr3.
> 
> Question is - do we really want this? While I provided aforementioned
> fix, I was never really convinced whether it was the hypervisor or
> the kernel that misbehaved here. It just was so that dealing with this
> on the hypervisor side was much easier.
> 
> Perhaps Keir would remember what the original intentions (if any)
> were regarding the page table layout (i.e. whether the above
> equality was really intended).

I think I did intend pt_base == cr3 == first page-table page in the
consecutive sequence of reserved pages on domain startup.

 -- Keir

> Jan
> 
>> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>> ---
>>  xen/include/public/xen.h | 12 +++++++++++-
>>  1 file changed, 11 insertions(+), 1 deletion(-)
>> 
>> diff --git a/xen/include/public/xen.h b/xen/include/public/xen.h
>> index 73fd7e7..4dd9615 100644
>> --- a/xen/include/public/xen.h
>> +++ b/xen/include/public/xen.h
>> @@ -693,7 +693,7 @@ typedef struct shared_info shared_info_t;
>>   *      c. list of allocated page frames [mfn_list, nr_pages]
>>   *         (unless relocated due to XEN_ELFNOTE_INIT_P2M)
>>   *      d. start_info_t structure        [register ESI (x86)]
>> - *      e. bootstrap page tables         [pt_base, CR3 (x86)]
>> + *      e. bootstrap page tables         [pt_base and CR3 (x86)]
>>   *      f. bootstrap stack               [register ESP (x86)]
>>   *  4. Bootstrap elements are packed together, but each is 4kB-aligned.
>>   *  5. The initial ram disk may be omitted.
>> @@ -705,6 +705,16 @@ typedef struct shared_info shared_info_t;
>>   *  8. There is guaranteed to be at least 512kB padding after the final
>>   *     bootstrap element. If necessary, the bootstrap virtual region is
>>   *     extended by an extra 4MB to ensure this.
>> + *
>> + * Note: Prior to 25833:bb85bbccb1c9. ("x86/32-on-64 adjust Dom0 initial
>> page
>> + * table layout") a bug caused the pt_base (3.e above) and cr3 to not point
>> + * to the start of the guest page tables (it was offset by two pages).
>> + * This only manifested itself on 32-on-64 dom0 kernels and not 32-on-64
>> domU
>> + * or 64-bit kernels of any colour. The page tables for a 32-on-64 dom0 got
>> + * allocated in the order: 'first L1','first L2', 'first L3', so the offset
>> + * to the page table base is by two pages back. The initial domain if it is
>> + * 32-bit and runs under a 64-bit hypervisor should _NOT_ use two of the
>> + * pages preceding pt_base and mark them as reserved/unused.
>>   */
>>  
>>  #define MAX_GUEST_CMDLINE 1024
>> -- 
>> 1.8.0.2
>> 
>> 
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> 
> 
>