Re: [Xen-devel] [PATCH RFC] arm64: 32-bit tolerant sync bitops

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 21/04/14 17:18, Vladimir Murzin wrote:
> On Thu, Apr 17, 2014 at 11:42 AM, David Vrabel <david.vrabel@xxxxxxxxxx> 
> wrote:
>> On 17/04/14 09:38, Vladimir Murzin wrote:
>>> Xen assumes that bit operations are able to operate on 32-bit size and
>>> alignment [1]. For arm64 bitops are based on atomic exclusive load/store
>>> instructions to guarantee that changes are made atomically. However, these
>>> instructions require that address to be aligned to the data size. Because, 
>>> by
>>> default, bitops operates on 64-bit size it implies that address should be
>>> aligned appropriately. All these lead to breakage of Xen assumption for 
>>> bitops
>>> properties.
>>>
>>> With this patch 32-bit sized/aligned bitops is implemented.
>>>
>>> [1] http://www.gossamer-threads.com/lists/xen/devel/325613
>>>
>>> Signed-off-by: Vladimir Murzin <murzin.v@xxxxxxxxx>
>>> ---
>>>  Apart this patch other approaches were implemented:
>>>  1. turn bitops to be 32-bit size/align tolerant.
>>>     the changes are minimal, but I'm not sure how broad side effect might be
>>>  2. separate 32-bit size/aligned operations.
>>>     it exports new API, which might not be good
>>
>> I've never been particularly happy with the way the events_fifo.c uses
>> casts for the sync_*_bit() calls and I think we should do option 2.
>>
>> A generic implementation could be something like:
>>
>> bool sync_test_bit32(uint32_t *v, unsigned bit)
>> {
>>      if (sizeof(unsigned long) == 8 && (unsigned long)v & 0x4)
>>          return sync_test_bit((unsigned long *)(v - 1), bit + 32);
>>      else
>>          return sync_test_bit((unsigned long *)v, bit);
>> }
>>
>> David
> 
> Talking about separate 32-bit ops I mean arch specific bitops which
> are targeting for 32-bit size/alignment.
> With separate API for Xen it looks awkward for me, because currently
> there are only a few users for sync_*_bit ops - Xen and HV.
> Xen assumes that these ops are 32 bit and looks like never try to deal
> with 64-bit (am I overlooking something?). So, sync ops are 32-bit
> de-facto, having separate version means there is support for both
> 32-bit and 64-bit ops, but (by now) there is no user for 64-bit ops.
> All this lead to obvious question why we need API conversion now? Is
> not it easier to turn assumptions to requirements?

Xen does use the sync bit ops on 64-bit values (for 2-level event channels).

David

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