On Wed, Apr 14, 2021 at 04:15:10PM +0100, Alexandru Elisei wrote: > Hi Drew, > > On 4/14/21 9:59 AM, Andrew Jones wrote: > > On Tue, Apr 13, 2021 at 05:34:24PM +0100, Alexandru Elisei wrote: > >> Hi Drew, > >> > >> On 4/7/21 7:59 PM, Andrew Jones wrote: > >>> Move secondary_entry helper functions out of .init and into .text, > >>> since secondary_entry isn't run at "init" time. > >> The tests aren't loaded using the loader, so as far as I can tell the reason for > >> having an .init section is to make sure the code from the start label is put at > >> offset 0 in the test binary. As long as the start label is kept at the beginning > >> of the .init section, and the loader script places the section first, I don't see > >> any issues with this change. > >> > >> The only hypothetical problem that I can think of is that the code from .init > >> calls code from .text, and if the text section grows very large we might end up > >> with a PC offset larger than what can be encoded in the BL instruction. That's > >> unlikely to happen (the offset is 16MB for arm and 64MB for arm64), and the .init > >> code already calls other functions (like setup) which are in .text, so we would > >> have this problem regardless of this change. And the compiler will emit an error > >> if that happens. > >> > >>> Signed-off-by: Andrew Jones <drjones@xxxxxxxxxx> > >>> --- > >>> arm/cstart.S | 62 +++++++++++++++++++++++++++----------------------- > >>> arm/cstart64.S | 22 +++++++++++------- > >>> 2 files changed, 48 insertions(+), 36 deletions(-) > >>> > >>> diff --git a/arm/cstart.S b/arm/cstart.S > >>> index d88a98362940..653ab1e8a141 100644 > >>> --- a/arm/cstart.S > >>> +++ b/arm/cstart.S > >>> @@ -96,32 +96,7 @@ start: > >>> bl exit > >>> b halt > >>> > >>> - > >>> -.macro set_mode_stack mode, stack > >>> - add \stack, #S_FRAME_SIZE > >>> - msr cpsr_c, #(\mode | PSR_I_BIT | PSR_F_BIT) > >>> - isb > >>> - mov sp, \stack > >>> -.endm > >>> - > >>> -exceptions_init: > >>> - mrc p15, 0, r2, c1, c0, 0 @ read SCTLR > >>> - bic r2, #CR_V @ SCTLR.V := 0 > >>> - mcr p15, 0, r2, c1, c0, 0 @ write SCTLR > >>> - ldr r2, =vector_table > >>> - mcr p15, 0, r2, c12, c0, 0 @ write VBAR > >>> - > >>> - mrs r2, cpsr > >>> - > >>> - /* first frame reserved for svc mode */ > >>> - set_mode_stack UND_MODE, r0 > >>> - set_mode_stack ABT_MODE, r0 > >>> - set_mode_stack IRQ_MODE, r0 > >>> - set_mode_stack FIQ_MODE, r0 > >>> - > >>> - msr cpsr_cxsf, r2 @ back to svc mode > >>> - isb > >>> - mov pc, lr > >>> +.text > >> Hm... now we've moved enable_vfp from .init to .text, and enable_vfp *is* called > >> from .init code, which doesn't fully match up with the commit message. Is the > >> actual reason for this change that the linker script for EFI will discard the > >> .init section? Maybe it's worth mentioning that in the commit message, because it > >> will explain this change better. > > Right, the .init section may not exist when linking with other linker > > scripts. I'll make the commit message more clear. > > > >> Or is it to align arm with arm64, where only > >> start is in the .init section? > >> > >>> > >>> enable_vfp: > >>> /* Enable full access to CP10 and CP11: */ > >>> @@ -133,8 +108,6 @@ enable_vfp: > >>> vmsr fpexc, r0 > >>> mov pc, lr > >>> > >>> -.text > >>> - > >>> .global get_mmu_off > >>> get_mmu_off: > >>> ldr r0, =auxinfo > >>> @@ -235,6 +208,39 @@ asm_mmu_disable: > >>> > >>> mov pc, lr > >>> > >>> +/* > >>> + * Vectors > >>> + */ > >>> + > >>> +.macro set_mode_stack mode, stack > >>> + add \stack, #S_FRAME_SIZE > >>> + msr cpsr_c, #(\mode | PSR_I_BIT | PSR_F_BIT) > >>> + isb > >>> + mov sp, \stack > >>> +.endm > >>> + > >>> +exceptions_init: > >>> + mrc p15, 0, r2, c1, c0, 0 @ read SCTLR > >>> + bic r2, #CR_V @ SCTLR.V := 0 > >>> + mcr p15, 0, r2, c1, c0, 0 @ write SCTLR > >>> + ldr r2, =vector_table > >>> + mcr p15, 0, r2, c12, c0, 0 @ write VBAR > >>> + > >>> + mrs r2, cpsr > >>> + > >>> + /* > >>> + * Input r0 is the stack top, which is the exception stacks base > >>> + * The first frame is reserved for svc mode > >>> + */ > >>> + set_mode_stack UND_MODE, r0 > >>> + set_mode_stack ABT_MODE, r0 > >>> + set_mode_stack IRQ_MODE, r0 > >>> + set_mode_stack FIQ_MODE, r0 > >>> + > >>> + msr cpsr_cxsf, r2 @ back to svc mode > >>> + isb > >>> + mov pc, lr > >>> + > >>> /* > >>> * Vector stubs > >>> * Simplified version of the Linux kernel implementation > >>> diff --git a/arm/cstart64.S b/arm/cstart64.S > >>> index 0a85338bcdae..d39cf4dfb99c 100644 > >>> --- a/arm/cstart64.S > >>> +++ b/arm/cstart64.S > >>> @@ -89,10 +89,12 @@ start: > >>> msr cpacr_el1, x4 > >>> > >>> /* set up exception handling */ > >>> + mov x4, x0 // x0 is the addr of the dtb > >> I suppose changing exceptions_init to use x0 as a scratch register instead of x4 > >> makes some sense if you look at it from the perspective of it being called from > >> secondary_entry, where all the functions use x0 as a scratch register. But it's > >> still called from start, where using x4 as a scratch register is preferred because > >> of the kernel boot protocol (x0-x3 are reserved). > >> > >> Is there an actual bug that this is supposed to fix (I looked for it and couldn't > >> figure it out) or is it just a cosmetic change? > > Now that exceptions_init isn't a private function of start (actually it > > hasn't been in a long time, considering secondary_entry calls it) I would > > like it to better conform to calling conventions. I guess I should have > > used x19 here instead of x4 to be 100% correct. Or, would you rather I > > just continue using x4 in exceptions_init in order to avoid the > > save/restore? > > To be honest, for this patch, I think it would be best to leave exceptions_init > unchanged: > > - We switch to using x0 like the rest of the code from secondary_entry, but > because of that we need to save and restore the DTB address from x0 in start, so I > don't think we've gained anything. > - It makes the diff larger. > - It runs the risk of introducing regressions (like all changes). > > Maybe this can be left for a separate patch that changes code called from C to > follow aapcs64. > OK, I'll switch it back to x4 and add a comment. Thanks, drew
