On April 1, 2026 7:59:17 AM PDT, Xin Li <[email protected]> wrote: > >>>>>>>> The existing 'sysret_rip' selftest asserts that 'regs->r11 == >>>>>>>> regs->flags'. This check relies on the behavior of the SYSCALL >>>>>>>> instruction on legacy x86_64, which saves 'RFLAGS' into 'R11'. >>>>>>>> However, on systems with FRED (Flexible Return and Event Delivery) >>>>>>>> enabled, instead of using registers, all state is saved onto the stack. >>>>>>>> Consequently, 'R11' retains its userspace value, causing the assertion >>>>>>>> to fail. >>>>>>>> Fix this by detecting if FRED is enabled and skipping the register >>>>>>>> assertion in that case. The detection is done by checking if the RPL >>>>>>>> bits of the GS selector are preserved after a hardware exception. >>>>>>>> IDT (via IRET) clears the RPL bits of NULL selectors, while FRED (via >>>>>>>> ERETU) preserves them. >>>>>>> I don't really like this. I think we have two credible choices: >>>>>>> 1. Define the Linux ABI to be that, on FRED systems, SYSCALL preserves >>>>>>> R11 and RCX on entry and exit. And update the test to actually test >>>>>>> this. >>>>>>> 2. Define the Linux ABI to be what it has been for quite a few years: >>>>>>> SYSCALL entry copies RFLAGS to R11 and RIP to RCX and SYSCALL exit >>>>>>> preserves all registers. >>>>>>> I'm in favor of #2. People love making new programming languages and >>>>>>> runtimes and inline asm and, these days, vibe coded crap. And it's >>>>>>> *easier* to emit a SYSCALL and forget to tell the compiler / code >>>>>>> generator that RCX and R11 are clobbered than it is to remember that >>>>>>> they're clobbered. And it's easy to test on FRED (well, not really, >>>>>>> but it hopefully will be some day) and it's easy to publish one's >>>>>>> code, and then everyone is a bit screwed when the resulting program >>>>>>> crashes sometimes on non-FRED systems. And it will be miserable to >>>>>>> debug. >>>>>>> (It's *really* *really* easy to screw this up in a way that sort of >>>>>>> works even on non-FRED: RCX and R11 are usually clobbered across >>>>>>> function calls, so one can get into a situation in which one's >>>>>>> generated code usually doesn't require that SYSCALL preserve one of >>>>>>> these registers until an inlining decision changes or some code gets >>>>>>> reordered, and then it will start failing. And making the failure >>>>>>> depend on hardware details is just nasty. >>>>>>> So I think we should add the ~2 lines of code to fix the SYSCALL entry >>>>>>> on FRED to match non-FRED. >>>>>> Yes; I'm afraid I have to concur. Preserving the clobber on entry for >>>>>> FRED systems is by far the safest choice. >>>>>> Aside from this selftest, fancy debuggers and anything that can transfer >>>>>> userspace state between machines might be 'surprised'. >>>>> Thanks Andy and Peter. >>>>> Indeed, making the selftest branch on FRED vs. non-FRED behavior >>>>> is not a good practice. The selftest should validate ABI consistency. >>>>> I agree with Andy's option #2, so this should be fixed in the FRED >>>>> syscall entry implementation. >>>>> Li Xin, does this direction look right to you? I can assit with >>>>> validation and keep the selftest aligned with the agreed ABI. >>>> Yes, consistency should take precedence over hardware-specific variations. >>>> I would like to hear from Andrew Cooper and hpa before we do it. >>> Per Andy’s suggestion, the change would be: >>> diff --git a/arch/x86/entry/entry_fred.c b/arch/x86/entry/entry_fred.c >>> index 88c757ac8ccd..a19898747a2c 100644 >>> --- a/arch/x86/entry/entry_fred.c >>> +++ b/arch/x86/entry/entry_fred.c >>> @@ -79,6 +79,9 @@ static __always_inline void fred_other(struct pt_regs >>> *regs) >>> { >>> /* The compiler can fold these conditions into a single test */ >>> if (likely(regs->fred_ss.vector == FRED_SYSCALL && regs->fred_ss.l)) { >>> + regs->cx = regs->ip; >>> + regs->r11 = regs->flags; >>> + >>> regs->orig_ax = regs->ax; >>> regs->ax = -ENOSYS; >>> do_syscall_64(regs, regs->orig_ax); >>> It adds 4 extra MOVs on this hot path, but I don’t see it's a problem here. >> >> We discussed this over a year ago, and at that point agreed that reserving >> the register was the desired behavior. Why has this changed now? > >Yes, that is technically simpler and cleaner. > >The question brought up by Andy is, is the RCX/R11 clobbering behavior an >established architectural contract, or is it an implementation detail that >software ignores? > >But both are hard to prove. > >I think Andy and PeterZ want to be on the safer side, i.e., this clobbering >behavior is established. >
I do see the point especially by the time developers will be mostly on FRED-capable hardware and their programs end up failing on legacy. I'm more annoyed because we actually had this discussion once already.
