Re: [PATCH] x86: Configure NX support earlier in setup_arch

From: Stuart Hayes
Date: Wed Jul 09 2014 - 20:56:44 EST

On 7/8/2014 5:38 PM, H. Peter Anvin wrote:

> On 07/08/2014 03:34 PM, Stuart Hayes wrote:
>> I haven't received any responses... is there a problem with the patch? Also CCing a couple people.
> I was on vacation last week and am still catching up.
> It would also help if you describe the real-world scenario that made you
> trip over this.
> -hpa

Well... I got this issue because a co-worker tripped over it. He had NX disabled in BIOS for some reason, and found that linux wouldn't boot--it hung right after grub2. I guess it took a while to figure out that it was the fact that NX was disabled that caused linux not to come up--and that could happen to other people. I don't know of any real-world scenarios in which someone would actually prefer to run a recent linux kernel with NX disabled, though.

It looks like some of the other boot paths into the kernel automatically clear the XD_DISABLE bit in the MISC_ENABLE MSR in the CPU (in verify_cpu), but that doesn't happen when grub2 jumps to startup_64 in arch/x86/boot/compressed/head_64.S. I guess instead of this patch, I could try to make a patch that turns NX back on (somewhere in startup_64), but since the kernel already supports NX being disabled, so I thought maybe just fixing that would be better. I didn't like seeing the kernel just die without giving any indication of what the problem is.


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