Re: [2637] 2009-10-16 Colin Watson <address@hidden>

[address@hidden]

On Sat, Oct 17, 2009 at 6:40 AM, Vladimir 'phcoder' Serbinenko
<address@hidden> wrote:
> Robert Millan wrote:
>> On Fri, Oct 16, 2009 at 03:38:44PM +0000, Colin Watson wrote:
>>
>>> Revision: 2637
>>>           http://svn.sv.gnu.org/viewvc/?view=rev&root=grub&revision=2637
>>> Author:   cjwatson
>>> Date:     2009-10-16 15:38:42 +0000 (Fri, 16 Oct 2009)
>>> Log Message:
>>> -----------
>>> 2009-10-16  Colin Watson  <address@hidden>
>>>
>>>      * configure.ac (TARGET_CFLAGS): Add -mno-mmx -mno-sse -mno-sse2
>>>      -mno-3dnow on x86 architectures.  Some toolchains enable these
>>>      features by default, but they rely on registers that aren't enabled
>>>      in GRUB.  Thanks to Vladimir Serbinenko for the suggestion.
>>>
>>
>> Would it be preferable to enable those registers instead?
>>
>>
> They are disabled for following simple reason:
> imagine an OS which isn't aware of those registers. Then if user
> simultaneously launches two applications which use those registers then
> they won't be preserved accross context switches leading to bugs. I
> don't know if there is a way to disable these registers again after
> enabling them. I don't think grub2 will benefit a lot because of

I think this cannot be accurate because virtual machines would be
badly broken (registers enabled by the host, guest without awareness
is exactly the situation you describe).


> mmx/sse/sse2/3dnow because we don't use floating point arithmetics. If I
> remember correctly sse has some commands for operations on parallel
> operations on integers but the only place I see where benefit could be
> considerable is decompression and decryption. If it's proven that

For example, memcpy is much faster using those extended instruction
sets.  Maybe that's not an important use case for grub2 either.

> decompressing/decrypting with SSE makes it considerably faster then
> enabling SSE would be a good thing. Anyhow for now we can just disable
> MMX/SSE/SSE2/3dnow and when someone implements handling those registers
> he can enable them again
> On FreeBSD kernel itself is compiled without MMX, SSE, SSE2 and 3dnow to
> avoid to have to handle them on kernel threads switches.
>
>
> --
> Regards
> Vladimir 'phcoder' Serbinenko
> Personal git repository: http://repo.or.cz/w/grub2/phcoder.git
>
>
>
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