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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 08.09.14 at 18:11, <JGross@xxxxxxxx> 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@xxxxxxxx>
>>> ---
>>>   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


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