On Wed, Jan 26, 2022 at 06:38:30AM +0000, Christophe Leroy wrote: > > > Le 25/01/2022 à 22:10, Luis Chamberlain a écrit : > > On Mon, Jan 24, 2022 at 09:22:34AM +0000, Christophe Leroy wrote: > >> This can also be useful on other powerpc/32 in order to maximize the > >> chance of code being close enough to kernel core to avoid branch > >> trampolines. > > > > Curious about all this branch trampoline talk. Do you have data to show > > negative impact with things as-is? > > See > https://github.com/linuxppc/linux/commit/2ec13df167040cd153c25c4d96d0ffc573ac4c40 > > Or > https://github.com/linuxppc/linux/commit/7d485f647c1f4a6976264c90447fb0dbf07b111d This was useful and fun to read, thanks. > > Also, was powerpc/32 broken then without this? The commit log seems to > > suggest so, but I don't think that's the case. How was this issue noticed? > > > Your question is related to the trampoline topic or the exec/noexec > flagging ? > > Regarding trampoline, everything is working OK. That's just cherry on > the cake, when putting data away you can have more code closer to the > kernel. But that would not have been a reason in itself for this series. > > Regarding the exec/noexec discussion, it's a real issue. powerpc/32 > doesn't honor page exec flag, so when you select STRICT_MODULES_RWX and > flag module data as no-exec, it remains executable. That's because > powerpc/32 MMU doesn't have a per page exec flag but only a per > 256Mbytes segment exec flag. > > Typical PPC32 layount: > 0xf0000000-0xffffffff : VMALLOC AREA ==> NO EXEC > 0xc0000000-0xefffffff : Linear kernel memory mapping > 0xb0000000-0xbfffffff : MODULES AREA ==> EXEC > 0x00000000-0xafffffff : User space ==> EXEC > > So STRICT_MODULES_RWX is broken on some powerpc/32 You know, this is the sort of information that I think would be very useful for the commit log. Can you ammend? > > > > Are there other future CPU families being planned where this is all true for > > as well? Are they goin to be 32-bit as well? > > Future I don't know. > > Regarding the trampoline stuff, I see at least the following existing > architectures with a similar constraint: > > ARM: > > https://elixir.bootlin.com/linux/v5.16/source/arch/arm/include/asm/memory.h#L55 > > ARM even has a config item to allow trampolines or not. I might add the > same to powerpc to reduce number of pages used by modules. > > https://elixir.bootlin.com/linux/v5.16/source/arch/arm/Kconfig#L1514 > > NDS32 has the constraint > > https://elixir.bootlin.com/linux/v5.16/source/arch/nds32/include/asm/memory.h#L41 > > NIOS2 has the constraint, allthough they handled it in a different way: > > https://elixir.bootlin.com/linux/v5.16/source/arch/nios2/kernel/module.c#L30 > > > > Even ARM64 benefits from modules closer to kernel: > > https://elixir.bootlin.com/linux/v5.16/source/arch/arm64/Kconfig#L1848 > > > Another future opportunity with the ability to allocate module parts > separately is the possibility to then use huge vmalloc mappings. > > Today huge vmalloc mappings cannot be used for modules, see recent > discussion at > https://patchwork.ozlabs.org/project/linuxppc-dev/patch/20211227145903.187152-4-wangkefeng.wang@xxxxxxxxxx/ Alrighty, this is sufficient information, thanks! Luis
