Re: [PATCH] eal/x86: fix build on systems with WAITPKG support

[Bruce Richardson <bruce.richardson@intel.com>]

On Mon, Aug 28, 2023 at 11:29:05AM +0200, David Marchand wrote:
> On Fri, Aug 25, 2023 at 5:29 PM Bruce Richardson
> <bruce.richardson@intel.com> wrote:
> >
> > When doing a build for a system with WAITPKG support and a modern
> > compiler, we get build errors for the "_umonitor" intrinsic, due to the
> > casting away of the "volatile" on the parameter.
> >
> > ../lib/eal/x86/rte_power_intrinsics.c: In function 'rte_power_monitor':
> > ../lib/eal/x86/rte_power_intrinsics.c:113:22: error: passing argument 1
> > of '_umonitor' discards 'volatile' qualifier from pointer target type
> > [-Werror=discarded-qualifiers]
> >   113 |         _umonitor(pmc->addr);
> >         |                   ~~~^~~~~~
> >
> > We can avoid this issue by using RTE_PTR_ADD(..., 0) to cast the pointer
> > through "uintptr_t" and thereby remove the volatile without warning.
> 
> As Morten, I prefer an explicit cast (keeping your comments) as it
> seems we are exploiting an implementation detail of RTE_PTR_ADD.
> 

Ok, I'll do a respin with explicit cast.

> 
> > We also ensure comments are correct for each leg of the
> > ifdef..else..endif block.
> 
> Thanks.. I had fixed other places but I have missed this one.
> 
> 
> >
> > Fixes: 60943c04f3bc ("eal/x86: use intrinsics for power management")
> > Cc: roretzla@linux.microsoft.com
> >
> > Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>
> > ---
> >  lib/eal/x86/rte_power_intrinsics.c | 12 ++++++------
> >  1 file changed, 6 insertions(+), 6 deletions(-)
> >
> > diff --git a/lib/eal/x86/rte_power_intrinsics.c b/lib/eal/x86/rte_power_intrinsics.c
> > index 4066d1392e..4f0404bfb8 100644
> > --- a/lib/eal/x86/rte_power_intrinsics.c
> > +++ b/lib/eal/x86/rte_power_intrinsics.c
> > @@ -103,15 +103,15 @@ rte_power_monitor(const struct rte_power_monitor_cond *pmc,
> >         rte_spinlock_lock(&s->lock);
> >         s->monitor_addr = pmc->addr;
> >
> > -       /*
> > -        * we're using raw byte codes for now as only the newest compiler
> > -        * versions support this instruction natively.
> > -        */
> > -
> >         /* set address for UMONITOR */
> >  #if defined(RTE_TOOLCHAIN_MSVC) || defined(__WAITPKG__)
> > -       _umonitor(pmc->addr);
> > +       /* use RTE_PTR_ADD to cast away "volatile" when using the intrinsic */
> > +       _umonitor(RTE_PTR_ADD(pmc->addr, 0));
> >  #else
> > +       /*
> > +        * we're using raw byte codes for compiler versions which
> > +        * don't support this instruction natively.
> > +        */
> >         asm volatile(".byte 0xf3, 0x0f, 0xae, 0xf7;"
> >                         :
> >                         : "D"(pmc->addr));
> 
> Tested-by: David Marchand <david.marchand@redhat.com>
> 
> An additional question, would Intel CI catch such issue?
> Or was it caught only because you are blessed with bleeding edge hw? :-)
> 
Not sure. I would hope so, though.

/Bruce