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

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2014-04-22 at 11:16 +0100, David Vrabel wrote:
> 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).

It is valid to use a 32-bit bitop on a 64-bit aligned bitmask though
(whereas the converse is not always true).

Ian.



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