On Tue, Oct 02, 2018 at 09:59:29AM +1000, Nicholas Piggin wrote: > On Mon, 1 Oct 2018 03:51:21 -0500 > Segher Boessenkool <seg...@kernel.crashing.org> wrote: > > And that is required by the ABI! > > > > """ > > 2.2.2.4. Protected Zone > > > > The 288 bytes below the stack pointer are available as volatile program > > storage that is not preserved across function calls. Interrupt handlers and > > any other functions that might run without an explicit call must take care > > to preserve a protected zone, also referred to as the red zone, of 512 bytes > > that consists of: > > > > * The 288-byte volatile program storage region that is used to hold saved > > registers and local variables > > * An additional 224 bytes below the volatile program storage region that is > > set aside as a volatile system storage region for system functions > > > > If a function does not call other functions and does not need more stack > > space than is available in the volatile program storage region (that is, 288 > > bytes), it does not need to have a stack frame. The 224-byte volatile system > > storage region is not available to compilers for allocation to saved > > registers and local variables. > > """ > > > > A routine has a red zone of 288 bytes. Below there is 224 more bytes of > > available storage, but that is not available to the routine itself: some > > (asynchronous) other code (like an interrupt) can use (i.e. clobber) it. > > Thanks Segher, that explains it very well and shows we are safe with > 288 in the kernel. So we can leave the code as-is, but the comment > could be updated. > > What are "system functions" exactly?
That is an excellent question. I think it was left vague on purpose? "Stuff a user program cannot do itself", "ABI stuff", "whatever the OS defines as system stuff"? > Can the kernel use that, or are > we talking about user mode system code like libraries? The kernel > could maybe use that for scratch space for synchronous interrupts to > avoid using a slow SPR for scratch. If you're already using the kernel stack, sure. When does this happen? Segher
