From mboxrd@z Thu Jan  1 00:00:00 1970
From: David Vrabel <david.vrabel@citrix.com>
Subject: Re: [PATCH] xen: fix alignment for bitops
Date: Tue, 15 Apr 2014 11:17:27 +0100
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To: Vladimir Murzin <murzin.v@gmail.com>
Cc: Ian Campbell <ian.campbell@citrix.com>, Stefano Stabellini <stefano.stabellini@eu.citrix.com>, Catalin Marinas <catalin.marinas@arm.com>, Will Deacon <will.deacon@arm.com>, david.vrabel@citrix.com, xen-devel@lists.xenproject.org, boris.ostrovsky@oracle.com
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On 14/04/14 21:43, Vladimir Murzin wrote:
> Bitops operations like set/clear/change mandate world aligned pointer, mainly
> because architectures specific implementation.
> 
> Looks that DEFINE_PER_CPU does required alignment for cpu_control_block;
> however, local copy used for bitops might not be world aligned.
> 
> Instruct compiler to keep local copy of "ready" world aligned.
[...]
> --- a/drivers/xen/events/events_fifo.c
> +++ b/drivers/xen/events/events_fifo.c
> @@ -285,7 +285,7 @@ static void consume_one_event(unsigned cpu,
>  static void evtchn_fifo_handle_events(unsigned cpu)
>  {
>  	struct evtchn_fifo_control_block *control_block;
> -	uint32_t ready;
> +	uint32_t __aligned(sizeof(long)) ready;

This doesn't look sufficient to me.  The event words within the event
array are 32-bit sized/aligned and sync_set_bit() and sync_clear_bit()
etc. are done on these words.

I think arm64 is going to have to provide sync_clear_bit(),
sync_set_bit(), sync_test_bit() and sync_test_and_set_bit() that work
with 32-bit sized and aligned words.

David