From: david.vrabel@citrix•com (David Vrabel)
To: linux-arm-kernel@lists•infradead.org
Subject: [Xen-devel] [PATCH RFC] arm64: 32-bit tolerant sync bitops
Date: Tue, 22 Apr 2014 11:16:47 +0100 [thread overview]
Message-ID: <5356418F.5090408@citrix.com> (raw)
In-Reply-To: <CABV+yWsFY4AnNa+Bs54ZNswCUga1OtFh3+5PPWHSEA4zRXJD3g@mail.gmail.com>
On 21/04/14 17:18, Vladimir Murzin wrote:
> On Thu, Apr 17, 2014 at 11:42 AM, David Vrabel <david.vrabel@citrix•com> 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@gmail•com>
>>> ---
>>> 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
next prev parent reply other threads:[~2014-04-22 10:16 UTC|newest]
Thread overview: 12+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-04-17 8:38 [PATCH RFC] arm64: 32-bit tolerant sync bitops Vladimir Murzin
2014-04-17 8:41 ` [PATCH] xen: use sync_clear_bit instead of clear_bit Vladimir Murzin
2014-04-17 10:23 ` David Vrabel
2014-04-17 10:42 ` [Xen-devel] [PATCH RFC] arm64: 32-bit tolerant sync bitops David Vrabel
2014-04-21 16:18 ` Vladimir Murzin
2014-04-22 10:16 ` David Vrabel [this message]
2014-04-22 10:55 ` Ian Campbell
2014-04-23 8:31 ` Ian Campbell
2014-04-22 20:56 ` Vladimir Murzin
2014-04-25 7:17 ` Ian Campbell
2014-04-25 8:43 ` Vladimir Murzin
2014-04-25 8:42 ` Vladimir Murzin
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