Date: Thu, 11 Jun 2020 16:02:30 -0500 From: Segher Boessenkool <segher@...nel.crashing.org> To: Nicholas Piggin <npiggin@...il.com> Cc: linuxppc-dev@...ts.ozlabs.org, libc-dev@...ts.llvm.org, musl@...ts.openwall.com, linux-api@...r.kernel.org Subject: Re: Linux powerpc new system call instruction and ABI Hi! On Thu, Jun 11, 2020 at 06:12:01PM +1000, Nicholas Piggin wrote: > Calling convention > ------------------ > The proposal is for scv 0 to provide the standard Linux system call ABI > with the following differences from sc convention: > > - lr is to be volatile across scv calls. This is necessary because the > scv instruction clobbers lr. From previous discussion, this should be > possible to deal with in GCC clobbers and CFI. > > - cr1 and cr5-cr7 are volatile. This matches the C ABI and would allow the > kernel system call exit to avoid restoring the volatile cr registers > (although we probably still would anyway to avoid information leaks). > > - Error handling: The consensus among kernel, glibc, and musl is to move to > using negative return values in r3 rather than CR0[SO]=1 to indicate error, > which matches most other architectures, and is closer to a function call. What about cr0 then? Will it be volatile as well (exactly like for function calls)? > Notes > ----- > - r0,r4-r8 are documented as volatile in the ABI, but the kernel patch as > submitted currently preserves them. This is to leave room for deciding > which way to go with these. The kernel has to set it to *something* that doesn't leak information ;-) Segher
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