On 4/26/17 7:55 AM, David Miller wrote:
From: Daniel Borkmann <dan...@iogearbox.net>
Date: Wed, 26 Apr 2017 10:14:42 +0200
On 04/26/2017 05:42 AM, Alexei Starovoitov wrote:
That sucks for sparc, of course. I don't have good suggestions for
workaround other than marking tcphdr as packed :(
From code efficiency point of view it still will be faster than
LD_ABS insn.
Plus, ld_abs would also only work on skbs right now, and would
need JIT support for XDP. But it's also cumbersome to use with
f.e. IPv6, etc, so should not be encouraged, imo.
One other option for !HAVE_EFFICIENT_UNALIGNED_ACCESS archs could
be to provide a bpf_skb_load_bytes() helper equivalent for XDP,
where you eventually do the memcpy() inside. We could see to inline
the helper itself to optimize it slightly.
We have to do something that works transparently and always,
regardless of whether HAVE_EFFICIENT_UNALIGNED_ACCESS is in
play or not.
And no, marking objects with __packed is not the answer.
I'm not suggesting to mark everything as __packed.
Why the following is not the answer?
index fd1e0832d409..c215dffd7189 100644
--- a/tools/testing/selftests/bpf/test_pkt_access.c
+++ b/tools/testing/selftests/bpf/test_pkt_access.c
@@ -19,13 +19,52 @@
#define barrier() __asm__ __volatile__("": : :"memory")
int _version SEC("version") = 1;
+struct __tcphdr {
+ __u16 source;
...
+ __u16 window;
+ __u16 check;
+ __u16 urg_ptr;
+}
+#if defined(__sparc__) || defined(__s390__)
+__packed
+#endif
+;
SEC("test1")
int process(struct __sk_buff *skb)
{
void *data_end = (void *)(long)skb->data_end;
void *data = (void *)(long)skb->data;
struct ethhdr *eth = (struct ethhdr *)(data);
- struct tcphdr *tcp = NULL;
+ struct __tcphdr *tcp = NULL;
__u8 proto = 255;
It's only needed for test_pkt_access.c test,
since it's being fancy and doing iph->ihl * 4.
Also such tcp->urg_ptr access into packed struct is more efficient
after JITing then sparc's own load_half_unaligned in asm, since it's
done inline and doesn't need a call.
Note that such tcphdr workaround is not necessary for more
real programs: test_l4lb.c and test_xdp.c, since they do:
if (iph->ihl != 5)
return drop;
Another idea:
x64, arm64, ppc have efficient unaligned.
s390 and mips64 have special instructions to do unaligned
access efficiently and we can make verifier convert unknown-align
load/stores into special internal load/stores, so they can be
executed differently by interpreter and by JITs.
Does sparc64 have some special instructions like that?
