Re: [dpdk-dev] [PATCH] test/bpf: use hton instead of __builtin_bswap

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> 
> Hi Konstantin,
> 
> At time of upstreaming the patch, I had compiled it on Cortex-A72 and it had worked with the -I parameter.

Just curious what is memory ordering on your box (BE/LE)?

> But when tried building it on
> x86-64, I encountered the error that you were facing. From my understanding of this problem, this is a portability issue. As you pointed out,
> gcc and clang implement __builtin_constant_p in a different way on x86 but that may not be true for ARM.
> 
> In such a case, to enable compilation on x86, we may want to write specific hton and ntoh functions. But this would become a tedious task
> because, in future, to enable compilation on other architectures besides x86 and ARM, we may have to repeat this process of writing
> analogs for each architecture.

Don't see why we need it done for each arch.
Obviously all is needed is BE and LE version.

> I think the best solution in the current scenario would be to use the -U parameter to enable compilation on
> x86 and later decide if we want to write a new patch with analogs for builtin hton() and ntoh().

I suppose -U__GNUC__ is ok.
If you decide to go that way, please update appropriate comments in *.c.

Konstantin

> 
> Best,
> Malvika
> 
> -----Original Message-----
> From: Ananyev, Konstantin <konstantin.ananyev@intel.com>
> Sent: Friday, October 19, 2018 7:17 AM
> To: Malvika Gupta <Malvika.Gupta@arm.com>
> Cc: dev@dpdk.org; Gavin Hu (Arm Technology China) <Gavin.Hu@arm.com>; Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>;
> nd <nd@arm.com>
> Subject: RE: [PATCH] test/bpf: use hton instead of __builtin_bswap
> 
> 
> Hi Malvika,
> 
> >
> > Hi Konstantin,
> >
> > I installed the clang version 4.0.1 to check for the issue you were
> > facing with -O2 compilation. I was able to compile with -O2 and -O0 optimization without any errors. Please see the exact command I
> used and the following output for your reference:
> >
> > $ clang -O2 -target bpf -I /usr/include/aarch64-linux-gnu/ -c t1.c $
> > clang -O0 -target bpf -I /usr/include/aarch64-linux-gnu/ -c t1.c $
> > clang -v clang version 4.0.1-10 (tags/RELEASE_401/final)
> > Target: aarch64-unknown-linux-gnu
> > Thread model: posix
> > InstalledDir: /usr/bin
> > Found candidate GCC installation:
> > /usr/bin/../lib/gcc/aarch64-linux-gnu/6
> > Found candidate GCC installation:
> > /usr/bin/../lib/gcc/aarch64-linux-gnu/6.4.0
> > Found candidate GCC installation:
> > /usr/bin/../lib/gcc/aarch64-linux-gnu/7
> > Found candidate GCC installation:
> > /usr/bin/../lib/gcc/aarch64-linux-gnu/7.3.0
> > Found candidate GCC installation:
> > /usr/bin/../lib/gcc/aarch64-linux-gnu/8
> > Found candidate GCC installation:
> > /usr/bin/../lib/gcc/aarch64-linux-gnu/8.0.1
> > Found candidate GCC installation: /usr/lib/gcc/aarch64-linux-gnu/6
> > Found candidate GCC installation: /usr/lib/gcc/aarch64-linux-gnu/6.4.0
> > Found candidate GCC installation: /usr/lib/gcc/aarch64-linux-gnu/7
> > Found candidate GCC installation: /usr/lib/gcc/aarch64-linux-gnu/7.3.0
> > Found candidate GCC installation: /usr/lib/gcc/aarch64-linux-gnu/8
> > Found candidate GCC installation: /usr/lib/gcc/aarch64-linux-gnu/8.0.1
> > Selected GCC installation: /usr/bin/../lib/gcc/aarch64-linux-gnu/7.3.0
> > Candidate multilib: .;@m64
> > Selected multilib: .;@m64
> >
> > I hope this was helpful to you.
> 
> Sorry for delay.
> I tried with clang 6.0, but still seeing same issue.
> Digging a bit more, I think the culprit is here:
> 
> #ifdef __GNUC__
> # if __GNUC__ >= 2
> #  define __bswap_16(x) \
>      (__extension__                                                           \
>       ({ unsigned short int __v, __x = (unsigned short int) (x);              \
>          if (__builtin_constant_p (__x))                                      \     <------ evaluates to FALSE !!!
>            __v = __bswap_constant_16 (__x);                                   \
>          else                                                                 \
>            __asm__ ("rorw $8, %w0"                                            \
>                     : "=r" (__v)                                              \
>                     : "0" (__x)                                               \
>                     : "cc");                                                  \
>          __v; }))
> # else
> /* This is better than nothing.  */
> #  define __bswap_16(x) \
>      (__extension__                                                           \
>       ({ unsigned short int __x = (unsigned short int) (x);                   \
>          __bswap_constant_16 (__x); }))
> # endif
> #else
> static __inline unsigned short int
> __bswap_16 (unsigned short int __bsx)
> {
>   return __bswap_constant_16 (__bsx);
> }
> #endif
> 
> Seems that gcc and clang implements __builtin_constant_p in a different way.
> Let say, the following function:
> int bbb1(void) {uint16_t x=0; return__bswap_constant_16(x);} when compiled with gcc would return 1, while when compiled with clang - 0.
> Adding 'const' to definition of 'x' cures the problem for clang.
> I don't know why there is no 'const' in original __bswap_16() macro.
> To overcome that problem, I used:
> clang -O2 -U__GNUC__ -target bpf -c t1.c
> 
> Another possible option - define our own analogs for buitin hton*().
> Konstantin
> 
> 
> > Best Regards,
> > Malvika Gupta
> >
> >
> > -----Original Message-----
> > From: Malvika Gupta
> > Sent: Wednesday, September 5, 2018 4:43 PM
> > To: Ananyev, Konstantin <konstantin.ananyev@intel.com>
> > Cc: dev@dpdk.org; Gavin Hu (Arm Technology China) <Gavin.Hu@arm.com>;
> > Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>; Brian Brooks
> > <Brian.Brooks@arm.com>; nd <nd@arm.com>
> > Subject: RE: [PATCH] test/bpf: use hton instead of __builtin_bswap
> >
> > Hi Ananyev,
> >
> > I used clang version 6.0.0. Please see the following output for your reference.
> >
> > $ clang -v
> > clang version 6.0.0-1ubuntu2 (tags/RELEASE_600/final)
> > Target: aarch64-unknown-linux-gnu
> > Thread model: posix
> > InstalledDir: /usr/bin
> > Found candidate GCC installation:
> > /usr/bin/../lib/gcc/aarch64-linux-gnu/6
> > Found candidate GCC installation:
> > /usr/bin/../lib/gcc/aarch64-linux-gnu/6.4.0
> > Found candidate GCC installation:
> > /usr/bin/../lib/gcc/aarch64-linux-gnu/7
> > Found candidate GCC installation:
> > /usr/bin/../lib/gcc/aarch64-linux-gnu/7.3.0
> > Found candidate GCC installation:
> > /usr/bin/../lib/gcc/aarch64-linux-gnu/8
> > Found candidate GCC installation:
> > /usr/bin/../lib/gcc/aarch64-linux-gnu/8.0.1
> > Found candidate GCC installation: /usr/lib/gcc/aarch64-linux-gnu/6
> > Found candidate GCC installation: /usr/lib/gcc/aarch64-linux-gnu/6.4.0
> > Found candidate GCC installation: /usr/lib/gcc/aarch64-linux-gnu/7
> > Found candidate GCC installation: /usr/lib/gcc/aarch64-linux-gnu/7.3.0
> > Found candidate GCC installation: /usr/lib/gcc/aarch64-linux-gnu/8
> > Found candidate GCC installation: /usr/lib/gcc/aarch64-linux-gnu/8.0.1
> > Selected GCC installation: /usr/bin/../lib/gcc/aarch64-linux-gnu/7.3.0
> > Candidate multilib: .;@m64
> > Selected multilib: .;@m64
> >
> > Also, the code compiles with both -O2 and -O0 for me.
> >
> > I hope this was helpful
> > Best,
> > Malvika
> >
> > -----Original Message-----
> > From: Ananyev, Konstantin <konstantin.ananyev@intel.com>
> > Sent: Tuesday, September 4, 2018 8:56 AM
> > To: Malvika Gupta <Malvika.Gupta@arm.com>
> > Cc: dev@dpdk.org; Gavin Hu (Arm Technology China) <Gavin.Hu@arm.com>;
> > Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>; Brian Brooks
> > <Brian.Brooks@arm.com>; nd <nd@arm.com>
> > Subject: RE: [PATCH] test/bpf: use hton instead of __builtin_bswap
> >
> > Hi,
> >
> > >
> > > Convert host machine endianness to networking endianness for
> > > comparison of incoming packets with BPF filter
> > >
> > >
> > > Signed-off-by: Malvika Gupta <malvika.gupta@arm.com>
> > > Reviewed-by: Gavin Hu <gavin.hu@arm.com>
> > > Reviewed-by: Brian Brooks <brian.brooks@arm.com>
> > > Suggested-by: Brian Brooks <brian.brooks@arm.com>
> > > ---
> > >  test/bpf/t1.c | 7 ++++---
> > >  test/bpf/t3.c | 3 ++-
> > >  2 files changed, 6 insertions(+), 4 deletions(-)
> > >
> > > diff --git a/test/bpf/t1.c b/test/bpf/t1.c index
> > > 60f9434ab..7943fcf34
> > > 100644
> > > --- a/test/bpf/t1.c
> > > +++ b/test/bpf/t1.c
> > > @@ -28,24 +28,25 @@
> > >  #include <net/ethernet.h>
> > >  #include <netinet/ip.h>
> > >  #include <netinet/udp.h>
> > > +#include <arpa/inet.h>
> > >
> > >  uint64_t
> > >  entry(void *pkt)
> > >  {
> > >  	struct ether_header *ether_header = (void *)pkt;
> > >
> > > -	if (ether_header->ether_type != __builtin_bswap16(0x0800))
> > > +	if (ether_header->ether_type != htons(0x0800))
> >
> > Which version of clang do you use?
> > With my one I get:
> > $ clang -O2 -target bpf -c t1.c
> > t1.c:37:34: error: couldn't allocate output register for constraint 'r'
> >         if (ether_header->ether_type != ntohs(0x0800))
> >                                         ^
> > /usr/include/netinet/in.h:402:21: note: expanded from macro 'ntohs'
> > #   define ntohs(x)     __bswap_16 (x)
> >                         ^
> > /usr/include/bits/byteswap-16.h:31:14: note: expanded from macro '__bswap_16'
> >            __asm__ ("rorw $8, %w0"
> >
> > With '-O0' it compiles ok.
> >
> > $ clang -v
> > clang version 4.0.1 (tags/RELEASE_401/final)
> > Target: x86_64-unknown-linux-gnu
> > Thread model: posix
> > InstalledDir: /usr/bin
> > Found candidate GCC installation:
> > /usr/bin/../lib/gcc/x86_64-redhat-linux/7
> > Found candidate GCC installation: /usr/lib/gcc/x86_64-redhat-linux/7
> > Selected GCC installation: /usr/bin/../lib/gcc/x86_64-redhat-linux/7
> > Candidate multilib: .;@m64
> > Candidate multilib: 32;@m32
> > Selected multilib: .;@m64
> >
> > Konstantin
> >
> > >  		return 0;
> > >
> > >  	struct iphdr *iphdr = (void *)(ether_header + 1);
> > >  	if (iphdr->protocol != 17 || (iphdr->frag_off & 0x1ffff) != 0 ||
> > > -			iphdr->daddr != __builtin_bswap32(0x1020304))
> > > +			iphdr->daddr != htonl(0x1020304))
> > >  		return 0;
> > >
> > >  	int hlen = iphdr->ihl * 4;
> > >  	struct udphdr *udphdr = (void *)iphdr + hlen;
> > >
> > > -	if (udphdr->dest !=  __builtin_bswap16(5000))
> > > +	if (udphdr->dest != htons(5000))
> > >  		return 0;
> > >
> > >  	return 1;
> > > diff --git a/test/bpf/t3.c b/test/bpf/t3.c index
> > > 531b9cb8c..24298b7c7
> > > 100644
> > > --- a/test/bpf/t3.c
> > > +++ b/test/bpf/t3.c
> > > @@ -17,6 +17,7 @@
> > >  #include <net/ethernet.h>
> > >  #include <rte_config.h>
> > >  #include "mbuf.h"
> > > +#include <arpa/inet.h>
> > >
> > >  extern void rte_pktmbuf_dump(FILE *, const struct rte_mbuf *,
> > > unsigned int);
> > >
> > > @@ -29,7 +30,7 @@ entry(const void *pkt)
> > >  	mb = pkt;
> > >  	eth = rte_pktmbuf_mtod(mb, const struct ether_header *);
> > >
> > > -	if (eth->ether_type == __builtin_bswap16(ETHERTYPE_ARP))
> > > +	if (eth->ether_type == htons(ETHERTYPE_ARP))
> > >  		rte_pktmbuf_dump(stdout, mb, 64);
> > >
> > >  	return 1;
> > > --
> > > 2.17.1