linux-kernel - Re: [PATCH LINUX v4] xen: event channel arrays are xen_ulong

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Date:	Tue, 19 Feb 2013 18:12:35 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Ian Campbell <ian.campbell@...rix.com>
CC:	"xen-devel@...ts.xen.org" <xen-devel@...ts.xen.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Jan Beulich <JBeulich@...e.com>,
	"Keir (Xen.org)" <keir@....org>, "Tim (Xen.org)" <tim@....org>,
	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH LINUX v4] xen: event channel arrays are xen_ulong_t and
 not unsigned long

On Tue, 19 Feb 2013, Ian Campbell wrote:
> On ARM we want these to be the same size on 32- and 64-bit.
> 
> This is an ABI change on ARM. X86 does not change.
> 
> Signed-off-by: Ian Campbell <ian.campbell@...rix.com>
> Cc: Jan Beulich <JBeulich@...e.com>
> Cc: Keir (Xen.org) <keir@....org>
> Cc: Tim Deegan <tim@....org>
> Cc: Stefano Stabellini <stefano.stabellini@...citrix.com>
> Cc: linux-arm-kernel@...ts.infradead.org
> Cc: xen-devel@...ts.xen.org
> Cc: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
> ---
> Changes since V3
>   s/read_evtchn_pending_sel/xchg_xen_ulong/ in a comment.
> Changes since V2
>   Add comments about the correct bitops to use, and on the ordering/barrier
>   requirements on xchg_xen_ulong.
> Changes since V1
>   use find_first_set not __ffs
>   fix some more unsigned long -> xen_ulong_t
>   use more generic xchg_xen_ulong instead of ...read_evtchn...

still doesn't apply to 3.8


>         do {
> -               unsigned long pending_words;
> +               xen_ulong_t pending_words;
> 
>                 vcpu_info->evtchn_upcall_pending = 0;
> 
>                 if (__this_cpu_inc_return(xed_nesting_count) - 1)
>                         goto out;
> 
> -#ifndef CONFIG_X86 /* No need for a barrier -- XCHG is a barrier on x86. */
> -               /* Clear master flag /before/ clearing selector flag. */
> -               wmb();
> -#endif
> -               pending_words = xchg(&vcpu_info->evtchn_pending_sel, 0);
> +               /*
> +                * Master flag must be /before/ clearing selector
> +                * flag. xchg_xen_ulong must contain an appropriate
> +                * barrier.
> +                */

Master flag must be *cleared* ...

> +               pending_words = xchg_xen_ulong(&vcpu_info->evtchn_pending_sel, 0);
> 
>                 start_word_idx = __this_cpu_read(current_word_idx);
>                 start_bit_idx = __this_cpu_read(current_bit_idx);
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