On 15/8/25 15:57, Peter Zijlstra wrote: > On Fri, Aug 15, 2025 at 08:42:39AM +0300, Alexei Starovoitov wrote: >> On Thu, Aug 14, 2025 at 2:17 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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. >> >> rax is used to pass tail_call count. >> See diagram in commit log: >> https://lore.kernel.org/all/20240714123902.32305-2-hffilwlqm@xxxxxxxxx/ >> Before that commit rax was used differently. >> Bottom line rax was used for a long time to support bpf_tail_calls. >> I'm traveling atm. So cc-ing folks for follow ups. > > IIRC the bpf2bpf tailcall doesn't use CFI at the moment. But let me > double check. > > So emit_cfi() is called at the very start of emit_prologue() and > __arch_prepare_bpf_trampoline() in the BPF_TRAMP_F_INDIRECT case. > > Now, emit_prologue() starts with the CFI bits, but the tailcall lands at > X86_TAIL_CALL_OFFSET, at which spot we only have EMIT_ENDBR(), nothing > else. So RAX should be unaffected at that point. > > So, AFAICT, we're good on that point. It is just the C level indirect > function call ABI that is affected, BPF internal conventions are > unaffected. > RAX is used for propagating tail_call_cnt_ptr from caller to callee for bpf2bpf+tailcall on x86_64. Before the aforementioned commit, RAX is used for propagating tail_call_cnt from caller to callee for the case. Thanks, Leon
