Re: [PATCH] expand x86 arch_shared_info to support >3 level p2m tree

[Andrew Cooper <andrew.cooper3@citrix.com>]

On 09/09/14 10:28, Jan Beulich wrote:
>>>> On 08.09.14 at 18:11, <JGross@suse.com> wrote:
>> On 09/08/2014 03:59 PM, Andrew Cooper wrote:
>>> On 08/09/14 14:48, Juergen Gross wrote:
>>>> The x86 struct arch_shared_info field pfn_to_mfn_frame_list_list
>>>> currently contains the mfn of the top level page frame of the 3 level
>>>> p2m tree, which is used by the Xen tools during saving and restoring
>>>> (and live migration) of pv domains. With three levels of the p2m tree
>>>> it is possible to support up to 512 GB of RAM for a pv domain.
>>> Specifically only 64bit PV domains have the 512GB limit.
>>>
>>> 32bit PV domains have a far larger supported RAM as they can fit twice
>>> as many mfns in each p2m page.
>>>
>>>>   To be
>>>> able to support more RAM an additional level is to be added.
>>>>
>>>> This patch expands struct arch_shared_info with a new p2m tree root
>>>> and the number of levels of the p2m tree. The new information is
>>>> indicated by the domain to be valid by storing ~0UL into
>>>> pfn_to_mfn_frame_list_list (this should be done only if more than
>>>> three levels are needed, of course).
>>>>
>>>> Signed-off-by: Juergen Gross <jgross@suse.com>
>>>> ---
>>>>   xen/include/public/arch-x86/xen.h | 8 +++++++-
>>>>   1 file changed, 7 insertions(+), 1 deletion(-)
>>>>
>>>> diff --git a/xen/include/public/arch-x86/xen.h 
>> b/xen/include/public/arch-x86/xen.h
>>>> index f35804b..b7fa2b6 100644
>>>> --- a/xen/include/public/arch-x86/xen.h
>>>> +++ b/xen/include/public/arch-x86/xen.h
>>>> @@ -224,7 +224,13 @@ struct arch_shared_info {
>>>>       /* Frame containing list of mfns containing list of mfns containing 
>> p2m. */
>>>>       xen_pfn_t     pfn_to_mfn_frame_list_list;
>>>>       unsigned long nmi_reason;
>>>> -    uint64_t pad[32];
>>>> +    /*
>>>> +     * Following two fields are valid if pfn_to_mfn_frame_list_list 
>> contains
>>>> +     * ~0UL.
>>>> +     */
>>>> +    unsigned long p2m_levels;   /* number of levels of p2m tree */
>>>> +    xen_pfn_t     p2m_root;     /* p2m tree top level mfn */
>>>> +    uint64_t pad[30];
>>> This padding is now wrong, as unsigned long is only 4 bytes in 32bit,
>>> and it is adjacent to another unsigned long.
>> The padding in this case is a nightmare.
>>
>> For 32 bits arch_shared_info_t is 64 bit aligned due to uint64_t pad[].
> No - uint64_t is 4-byte aligned on 32-bit.
>
>> This enforces a 4 byte hole before the structure (there are 3 4-byte
>> fields before it in shared_info_t). And before pad[] there is another
>> 4-byte hole.
>>
>> And don't forget: on 32 bits xen_pfn_t is 4 bytes, too. I could either
>> align each new variable explicitly to 8 bytes or I could use a union.
>>
>> As arch_shared_info_t is at the end of shared_info_t I could just ignore
>> the alignment/padding ...
>>
>> What is your preference?
> As I view it, the exact padding size here doesn't really matter: The
> shared info lives in a separate page anyway.
>
> Jan
>

Which begs the question why the padding exists in the first place?

arch_shared_info is the final field in shared_info, and shared_info is
explicitly documented not to necessarily have a consistent size across
different versions of Xen.

Frankly, I think the padding can just be removed.  It serves no purpose,
and attempting to maintain it correctly is proving very difficult.

~Andrew