Thanks! Xin
> On Mar 31, 2026, at 8:15 PM, H. Peter Anvin <[email protected]> wrote: > > On March 31, 2026 6:59:06 PM PDT, Xin Li <[email protected]> wrote: >> >> >>>> On Mar 30, 2026, at 11:03 PM, 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 cleaner. The question is, is the RCX/R11 clobbering behavior an established architectural contract, or is it an implementation detail that software ignores? I think Andy and Peter want to be on the safer side, which kind of assumes that this is established.
