linux-kernel - Re: x86-64: memset()/memcpy() not fully standards compliant

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Message-ID: <20120105182846.GQ11715@one.firstfloor.org>
Date:	Thu, 5 Jan 2012 19:28:46 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Jan Beulich <JBeulich@...e.com>
Cc:	mingo@...e.hu, tglx@...utronix.de, hpa@...or.com,
	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org
Subject: Re: x86-64: memset()/memcpy() not fully standards compliant

On Thu, Jan 05, 2012 at 02:04:30PM +0000, Jan Beulich wrote:
> Forever these two functions have been limited to deal with at most 4G
> at a time. While I cannot point out an in-tree user that would require
> larger sizes, it is now the second time that within our Xen kernel we got
> bitten by that limitation. Would you nevertheless accept a patch to
> eliminate those shortcomings (iirc there may need to be workarounds
> for CPU bugs when it comes to using string instructions on such large
> blocks, albeit memmove() doesn't seem to care)?
> 
> Otherwise, is there any rationale for this sort of lurking bug?

Most (all?) of the CPUs I cared about when writing that code had 
bugs with string instructions and >4GB.

And I don't think anything in the kernel would ever process that big
areas in a single operation.

So it never seemed worth caring about.

You probably would need at least one more CMP & conditional jump
in the hot path, so everyone would pay for something that never happens.

-Andi
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