Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation

[Joao Moreira]

> Clang always uses r11 for the indirect call with retpolines, so we'd
> need to use another register. Nevertheless, splitting the constant into
> two instructions would solve the call target gadget issue.

Yeah, it clicked later yesterday. But, FWIIW, R10 is also considered a 
scratch register, although used for passing static chain pointers which 
I think is not a thing in kernel context. Last case scenario we can 
always do liveness analysis and I doubt we'll have a significant (if 
any) number of spills.

If we are comparing through registers, I would suggest using a sub 
instruction instead of a cmp, as this will destroy the contents of the 
register and prevent it from being re-used on further unprotected 
indirect branches, if any exists.