From owner-freebsd-hackers@FreeBSD.ORG  Mon Dec 31 12:30:45 2007
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Date: Mon, 31 Dec 2007 20:30:35 +0800
From: Erich Dollansky <oceanare@pacific.net.sg>
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To: Kostik Belousov <kostikbel@gmail.com>
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Cc: Kip Macy <kip.macy@gmail.com>, Ivan Voras <ivoras@freebsd.org>,
	freebsd-hackers@freebsd.org
Subject: Re: Architectures with strict alignment?
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Hi,

Kostik Belousov wrote:
> On Mon, Dec 31, 2007 at 05:38:43PM +0800, Erich Dollansky wrote:
>> Kostik Belousov wrote:
>>> On Sat, Dec 29, 2007 at 01:12:04PM +0200, Kostik Belousov wrote:
>>>> On Sat, Dec 29, 2007 at 12:14:11AM -0800, Kip Macy wrote:
>>> I.e., it seems that gcc does not feel too guilty generating unaligned
>>> half-word writes on i386. :(
>> this should not be a problem inside a cache line.
>>
>> If the access goes accross two cache lines and the other cache line is 
>> not in the cache, it becomes real difficult.
>>
>> I can't tell you what the hardware actually does in this case.
>>
>> It should read the second affected cache line into the cache. But what 
>> happens if the second affected cache line is blocked by another CPU 
>> while the current CPU blocks the first cache line?
> 
> From the manual, 253668, 7.1.1:
> 
> I think we might get any half of the operation as a result.

so, both CPUs are blocked as none can access the other cache line.

Is there really nothing in a normal PC to handle this?

I do not know. The hardware I developed earlier was able to handle this 
by aborting both bus cycles.

It was then task of the operating system to handle this.

Erich