Re: [PATCH] arm64: write_sysreg asm illegal for aarch32

[Mark Salyzyn]

On 11/01/2017 11:16 AM, Mark Salyzyn wrote:
> On 11/01/2017 10:56 AM, Mark Rutland wrote:
> > On Wed, Nov 01, 2017 at 10:49:00AM -0700, Mark Salyzyn wrote:
> > > On 11/01/2017 10:14 AM, Robin Murphy wrote:
> > > > On 01/11/17 16:58, Mark Salyzyn wrote:
> > > > > Cross compiling to aarch32 (for vdso32) using clang correctly
> > > > > identifies that (the unused) write_sysreg inline asm directive is
> > > > > illegal in that architectural context:
> > > > >
> > > > > arch/arm64/include/asm/arch_timer.h: error: invalid input 
> > > > > constraint 'rZ' in asm
> > > > > ÂÂÂÂÂÂÂÂÂ write_sysreg(cntkctl, cntkctl_el1);
> > > > > ÂÂÂÂÂÂÂÂÂ ^
> > > > > arch/arm64/include/asm/sysreg.h: note: expanded from macro 
> > > > > 'write_sysreg'
> > > > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ : : "rZ" (__val));
> > > > > ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ ^
> > > > >
> > > > > GCC normally checks for correctness everywhere. But uniquely for
> > > > > unused asm, will optimize out and suppress the error report.
> > > > It sounds more like some paths are wrong in the compat vDSO build if
> > > > it's pulling in this header in the first place - nothing in this 
> > > > file is
> > > > relevant to AArch32.
> > > >
> > > > Robin.
> > > And yet, when you CROSS_COMPILE_ARM32 a vdso32, you have no choice 
> > > but to
> > > utilize the arm64 headers since they contain all the relevant kernel
> > > structures and environment.
> > This itself is the underlying issue.
> >
> > When building the compat VDSO, we must ensure that we only include
> > headers that make sense for 32-bit arm.
> >
> > If the build system can't do that today, we should rework it so that it
> > can. Anything else cannot be a complete fix.
> >
> > > asm/arch_timer.h (remember we are using arm instructions to access 
> > > arch64
> > > timers)
> > >
> > > linux/time.h (really only for struct timespec())
> > >
> > > asm/processor.h (eg: cpu_relax())
> > >
> > > pull in a _lot_ of architectural related cruft that always somehow 
> > > picks up
> > > asm/sysreg.h somewhere in the multitude of includes to fulfill some 
> > > unused
> > > inline's needs.
> > ... these are just the particular symptoms this problem results in
> > today.
> >
> > Thanks,
> > Mark.
>
> Ok, will attack it and see just how bad the scale is...
>
> . . .
>
> -- Mark
Scoped, not as bad as I thought, but there is some open-coded evilness 
to fix:

1) linux/jiffies.h can not be included, replace with open coding:

#include <asm/param.h>

#define TICK_NSEC TICK_NSEC ((NSEC_PER_SEC+HZ/2)/HZ)

2) linux/hrtimer.h can not be included, replace with open coding (must 
have above to work):

#define LOW_RES_NSECÂÂÂ ÂÂÂ TICK_NSEC
#ifdef CONFIG_HIGH_RES_TIMERS
# define HIGH_RES_NSECÂÂÂ ÂÂÂ 1
# define MONOTONIC_RES_NSECÂÂÂ HIGH_RES_NSEC
#else
# define MONOTONIC_RES_NSECÂÂÂ LOW_RES_NSEC
#endif

3) asm/processor.h can not be included, replace with open coding:

static inline void cpu_relax(void)
{
ÂÂÂ asm volatile("yield" ::: "memory");
}

4) linux/time.h can not be included, replace with open coding:

#include <linux/compiler.h>

#include <linux/math64.h>

#include <uapi/linux/time.h>

#define NSEC_PER_SEC 1000000000L

static __always_inline void timespec_add_ns(struct timespec *a, u64 ns)
{
ÂÂÂ a->tv_sec += __iter_div_u64_rem(a->tv_nsec + ns, NSEC_PER_SEC, &ns);
ÂÂÂ a->tv_nsec = ns;
}


I am at a loss to determine if there is an acceptable way to split off 
the open-coding. For instance asm-generic/processor.h (for cpu_relax()), 
uapi/linux/hrtimer.h and uapi/linux/jiffies.h for the #defines (uapi is 
a bad choice, flipping coin?). The time open-coding is probably OK given 
that they have not changed since near the limits of git history.

-- Mark