Re: [PATCH kvm-unit-tests 1/8] arm/arm64: Reorganize cstart assembler

[Andrew Jones <drjones@redhat.com>]

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@redhat.com>
> > ---
> >  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?

Thanks,
drew