Small Ivy features: FSGSBASE and SMEP.

Michael Fuckner michael at
Sun Sep 9 06:42:40 UTC 2012

Hi all,

I changed your patch slightly to apply to specialreh.h on STABLE

root at c64:/root # diff smep.1.patch.bak smep.1.patch
< diff --git a/sys/x86/include/specialreg.h b/sys/x86/include/specialreg.h
 > diff --git a/sys/amd64/include/specialreg.h 
< --- a/sys/x86/include/specialreg.h
< +++ b/sys/x86/include/specialreg.h
 > --- a/sys/amd64/include/specialreg.h
 > +++ b/sys/amd64/include/specialreg.h

I got a new kernel, but it is stuck immediately (kerneltrap 9 with 
interrupts disabled), system doesn't boot on E3-1230 V2 on Supermicro 

Anything else I could check?


On 09/08/2012 08:10 PM, Konstantin Belousov wrote:
> Please find at
> the patch which should enable the FSGSBASE and SMEP features
> supposedly present in the IvyBridge CPUs.
> FSGSBASE are four new instructions available in the 64bit mode only.
> They allow to access bases for %fs and %gs without touching MSRs.
> This makes it possible to both read and write bases in the user mode,
> or in ring 0 with lower overhead.
> At the moment, WRFSBASE/WRGSBASE instructions should work, but are
> useless since any interrupt or context switch overrides bases with the
> values set by the arch syscall. Still, RDFSBASE/RDGSBASE might be useful
> for some code and I see no reason not to enable them.
> SMEP is the nice feature of the processor which makes it trap if ring
> 0 tries to execute an instruction from usermode-accessible page. It is
> another mitigation for things like calling user-controllable function
> pointer in kernel, as well as a protection for NULL function pointer
> dereference.
> I am sure that we never execute anything in kernel from user page, but
> I did not tested the patch since I have no Ivy machine.
> I need your reports about boot on Ivy with patch applied. Please include
> the lines from verbose dmesg with CPU Features. In particular, the
> 'Standard Extended Features' report should appear in output.
> Thanks.

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