Re: [RFC][PATCH] x86,ibt: Use UDB instead of 0xEA

[Peter Zijlstra]

On Thu, Aug 14, 2025 at 06:27:44PM -0700, H. Peter Anvin wrote:
> On 2025-08-14 04:17, Peter Zijlstra wrote:
> > Hi!
> > 
> > A while ago FineIBT started using the instruction 0xEA to generate #UD.
> > All existing parts will generate #UD in 64bit mode on that instruction.
> > 
> > However; Intel/AMD have not blessed using this instruction, it is on
> > their 'reserved' list for future use.
> > 
> > Peter Anvin worked the committees and got use of 0xD6 blessed, and it
> > will be called UDB (per the next SDM or so).
> > 
> > Reworking the FineIBT code to use UDB wasn't entirely trivial, and I've
> > had to switch the hash register to EAX in order to free up some bytes.
> > 
> > Per the x86_64 ABI, EAX is used to pass the number of vector registers
> > for varargs -- something that should not happen in the kernel. More so,
> > we build with -mskip-rax-setup, which should leave EAX completely unused
> > in the calling convention.
> > 
> > The code boots and passes the LKDTM CFI_FORWARD_PROTO test for various
> > combinations (non exhaustive so far).
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> 
> Looks good to me (and using %eax will save one byte per call site as
> well), but as per our IRC discussion, *my understanding* is that the
> best possible performance (least branch predictor impact across
> implementations) is to use a forward branch with a 2E prefix (jcc,pn in
> GAS syntax) rather than a reverse branch, if space allows.

Oh right. I did see that comment on IRC and them promptly forgot about
it again :/ I'll have a poke. Scott, do you agree? You being responsible
for the backward jump and such.