linux-kernel - Re: IO space memcpy support for userspace.

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Date:	Sat, 6 Dec 2008 16:34:43 +1000
From:	"Dave Airlie" <airlied@...il.com>
To:	"David Miller" <davem@...emloft.net>
Cc:	carlos@...temhalted.org, linux-kernel@...r.kernel.org,
	libc-alpha@...rces.redhat.com
Subject: Re: IO space memcpy support for userspace.

On Sat, Dec 6, 2008 at 6:22 AM, David Miller <davem@...emloft.net> wrote:
> From: "Carlos O'Donell" <carlos@...temhalted.org>
> Date: Fri, 5 Dec 2008 12:32:04 -0500
>
>> On Thu, Dec 4, 2008 at 10:40 PM, Dave Airlie <airlied@...il.com> wrote:
>> > I'm sure this has come up before and I'm sure I'll either wish I never
>> > posted this or someone will show me the crisp corpse of the last guy
>> > who suggested it.
>>
>> Do you plan to prevent the compiler from issuing the same sorts of
>> instructions that might appear in an optimized memcpy?
>>
>> Isn't it dangerous to have memory that doesn't behave like normal
>> memory, and yet try to treat it like normal memory?
>>
>> This mismatch of abstractions is a warning that must not be ignored.
>
> This is basically my opinion as well.
>
> You'll pretty much need to surround accesses to these places with
> accessor macros that do whatever is necessary on a given platform and
> avoids the "dangerous" instructions in cases like IA64.
>
> Treating them like normal memory isn't going to work on all systems.

Its a real pain in the ass with dynamic buffer objects, we don't want userspace
to care where they are located, the kernel migrates them in/out of
video memory, GART, local RAM etc.

However I suspect I just need on these platforms to ban any CPU
accesses to pixmaps in VRAM. However
sw fallbacks to the front buffer will always need these accesses.

Its going to be a real pain getting any traction this stuff upstream
(X.org/Mesa) where the world is x86 and maybe the odd powerpc, having
to do special accessors for shithouse hw is never going to be fun.

Maybe I should start libshithouse to encapsulate the problem, I'll
think about it some more.

Dave.

> BTW, the sunffb xorg driver has special code for "graphics copy"
> which is essentially just a scanline by scanline GCOPY using the
> MMX like stuff sparc64 has.  It also is mindful of avoiding access
> patterns that are known to lock up that chip :)
>
> That's just an aside, since sunffb doesn't provide any offscreen
> pixmap memory and thus shouldn't be susceptible to this problem being
> discussed here.
>
>
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