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From:   Joao Moreira <joao@overdrivepizza.com>
To:     Sami Tolvanen <samitolvanen@google.com>
Cc:     Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
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        Alexei Starovoitov <ast@kernel.org>,
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        Peter Collingbourne <pcc@google.com>
Subject: Re: [patch 00/38] x86/retbleed: Call depth tracking mitigation
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> 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.