On Mon, Jan 06, 2020 at 10:41:55AM +0000, Alexandru Elisei wrote: > On 1/2/20 6:11 PM, Andre Przywara wrote: > > On Tue, 31 Dec 2019 16:09:37 +0000 > > Alexandru Elisei <alexandru.elisei@xxxxxxx> wrote: > >> +.global asm_cpu_psci_cpu_die > >> +asm_cpu_psci_cpu_die: > >> + ldr r0, =PSCI_0_2_FN_CPU_OFF > >> + hvc #0 > >> + b . > > I am wondering if this implementation is actually too simple. Both > > the current implementation and the kernel clear at least the first > > three arguments to 0. > > I failed to find a requirement for doing this (nothing in the SMCCC > > or the PSCI spec), but I guess it would make sense when looking at > > forward compatibility. > > The SMC calling convention only specifies the values for the arguments that are > used by a function, not the values for all possible arguments. kvm-unit-tests sets > the other arguments to 0 because the function prototype that does the actual SMC > call takes 4 arguments. The value 0 is a random value that was chosen for those > unused parameters. For example, it could have been a random number on each call. That's correct. A caller can leave arbitrary values in non-argument registers, in the same manner as a caller of an AAPCS function. The callee should not consume those values as they are not arguments. > Let me put it another way. Suggesting that unused arguments should be set to 0 is > the same as suggesting that normal C function that adheres to procedure call > standard for arm64 should always have 8 arguments, and for a particular function > that doesn't use all of them, they should be set to 0 by the caller. Heh, same rationale. :) > @Mark Rutland has worked on the SMC implementation for the Linux kernel, if he > wants to chime in on this. > > Thanks, > Alex > > At the very least it's a change in behaviour (ignoring the missing printf). > > So shall we just clear r1, r2 and r3 here? (Same for arm64 below) There's no need to zero non-argument registers, and it could potentially mask bugs in callees, so I don't think it's a good idea to do so. If you really want to test that the callee is robust and correct, it would be better to randomize the content of non-argument regsiters to fuzz the callee. Thanks, Mark.
