Messages in this thread | | | Date | Sun, 06 Dec 2009 16:22:09 -0800 | From | "H. Peter Anvin" <> | Subject | Re: Kexec failure on RDC (and possibly other early x86) platforms |
| |
On 12/06/2009 08:52 AM, Avi Kivity wrote: > On 12/06/2009 01:33 PM, bifferos wrote: >> I've updated the patch to allow Kexec to work on RDC platforms >> here: >> http://bifferboard.svn.sourceforge.net/viewvc/bifferboard/slack/kernel/2.6.32/0003-kexec-fix.patch >> >> >> I'm curious as to how one can detect the presence of the CR4 >> register in assembler, as I'm sure it's possible, but I've just >> #ifndefed out the offending instruction in this patch and it >> seems to work with this change. >> > > One way is to execute the instruction and trap the #UD exception if it > is not supported. Not sure whether you have an IDT set up or whether > your cpu traps on mov cr4. > > If your cpu supports cpuid you can test for features that indicate bits > in cr4 are available, for example VME, DE, PSE, PVI, and PAE. If none > are available you likely don't have cr4 (and even if you do, it's > pointless to reset it). >
In general, I believe, existence of CPUID == existence of CR4.
(There were some late 486's which had CPUID, I believe they also had CR4.) The first-principles test of catching the trap is more direct, though. Inside the kernel we have read_cr4_safe() for that.
-hpa
-- H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf.
| |