On Tue, Jan 17, 2023 at 04:57:03PM +0300, Kirill A. Shutemov wrote:
> On Tue, Jan 17, 2023 at 02:05:22PM +0100, Peter Zijlstra wrote:
> > On Wed, Jan 11, 2023 at 03:37:27PM +0300, Kirill A. Shutemov wrote:
> >
> > > #define __untagged_addr(untag_mask, addr)
> > > u64 __addr = (__force u64)(addr); \
> > > - s64 sign = (s64)__addr >> 63; \
> > > - __addr &= untag_mask | sign; \
> > > + if (static_branch_likely(&tagged_addr_key)) { \
> > > + s64 sign = (s64)__addr >> 63; \
> > > + __addr &= untag_mask | sign; \
> > > + } \
> > > (__force __typeof__(addr))__addr; \
> > > })
> > >
> > > #define untagged_addr(addr) __untagged_addr(current_untag_mask(), addr)
> >
> > Is the compiler clever enough to put the memop inside the branch?
>
> Hm. You mean current_untag_mask() inside static_branch_likely()?
>
> But it is preprocessor who does this, not compiler. So, yes, the memop is
> inside the branch.
>
> Or I didn't understand your question.
Nah, call it a pre-lunch dip, I overlooked the whole CPP angle -- d'0h.
That said, I did just put it through a compiler to see wth it did and it
is pretty gross:
GCC-12.2:
0000 00000000000023b0 <write_ok_or_segv>:
0000 23b0: 48 89 fa mov %rdi,%rdx
0003 23b3: eb 76 jmp 242b <write_ok_or_segv+0x7b>
0005 23b5: 65 48 8b 0d 00 00 00 00 mov %gs:0x0(%rip),%rcx # 23bd <write_ok_or_segv+0xd> 23b9: R_X86_64_PC32 tlbstate_untag_mask-0x4
000d 23bd: 48 89 f8 mov %rdi,%rax
0010 23c0: 48 c1 f8 3f sar $0x3f,%rax
0014 23c4: 48 09 c8 or %rcx,%rax
0017 23c7: 48 21 f8 and %rdi,%rax
001a 23ca: 48 b9 00 f0 ff ff ff 7f 00 00 movabs $0x7ffffffff000,%rcx
0024 23d4: 48 39 f1 cmp %rsi,%rcx
0027 23d7: 72 14 jb 23ed <write_ok_or_segv+0x3d>
0029 23d9: 48 29 f1 sub %rsi,%rcx
002c 23dc: be 01 00 00 00 mov $0x1,%esi
0031 23e1: 48 39 c1 cmp %rax,%rcx
0034 23e4: 72 07 jb 23ed <write_ok_or_segv+0x3d>
0036 23e6: 89 f0 mov %esi,%eax
0038 23e8: e9 00 00 00 00 jmp 23ed <write_ok_or_segv+0x3d> 23e9: R_X86_64_PLT32 __x86_return_thunk-0x4
003d 23ed: 65 48 8b 04 25 00 00 00 00 mov %gs:0x0,%rax 23f2: R_X86_64_32S pcpu_hot
0046 23f6: 48 89 90 68 0b 00 00 mov %rdx,0xb68(%rax)
004d 23fd: be 01 00 00 00 mov $0x1,%esi
0052 2402: bf 0b 00 00 00 mov $0xb,%edi
0057 2407: 48 c7 80 78 0b 00 00 06 00 00 00 movq $0x6,0xb78(%rax)
0062 2412: 48 c7 80 70 0b 00 00 0e 00 00 00 movq $0xe,0xb70(%rax)
006d 241d: e8 00 00 00 00 call 2422 <write_ok_or_segv+0x72> 241e: R_X86_64_PLT32 force_sig_fault-0x4
0072 2422: 31 f6 xor %esi,%esi
0074 2424: 89 f0 mov %esi,%eax
0076 2426: e9 00 00 00 00 jmp 242b <write_ok_or_segv+0x7b> 2427: R_X86_64_PLT32 __x86_return_thunk-0x4
007b 242b: 48 89 f8 mov %rdi,%rax
007e 242e: eb 9a jmp 23ca <write_ok_or_segv+0x1a>
Note the stupid jump to the end and back. Not all sites do this mind
you, but a fair number seem to do it.
Let me try llvm to see if it is any smarter.
CLANG-16:
0000 0000000000002d50 <write_ok_or_segv>:
0000 2d50: 41 57 push %r15
0002 2d52: 41 56 push %r14
0004 2d54: 41 54 push %r12
0006 2d56: 53 push %rbx
0007 2d57: 48 89 f3 mov %rsi,%rbx
000a 2d5a: 48 89 fa mov %rdi,%rdx
000d 2d5d: 49 89 fe mov %rdi,%r14
0010 2d60: eb 15 jmp 2d77 <write_ok_or_segv+0x27>
0012 2d62: 48 89 d0 mov %rdx,%rax
0015 2d65: 48 c1 f8 3f sar $0x3f,%rax
0019 2d69: 65 4c 8b 35 00 00 00 00 mov %gs:0x0(%rip),%r14 # 2d71 <write_ok_or_segv+0x21> 2d6d: R_X86_64_PC32 tlbstate_untag_mask-0x4
0021 2d71: 49 09 c6 or %rax,%r14
0024 2d74: 49 21 d6 and %rdx,%r14
0027 2d77: f3 0f 1e fa endbr64
002b 2d7b: 49 bf 00 f0 ff ff ff 7f 00 00 movabs $0x7ffffffff000,%r15
0035 2d85: 4d 89 fc mov %r15,%r12
0038 2d88: 49 29 dc sub %rbx,%r12
003b 2d8b: 72 05 jb 2d92 <write_ok_or_segv+0x42>
003d 2d8d: 4d 39 f4 cmp %r14,%r12
0040 2d90: 73 34 jae 2dc6 <write_ok_or_segv+0x76>
0042 2d92: 65 48 8b 05 00 00 00 00 mov %gs:0x0(%rip),%rax # 2d9a <write_ok_or_segv+0x4a> 2d96: R_X86_64_PC32 pcpu_hot-0x4
004a 2d9a: 48 c7 80 78 0b 00 00 06 00 00 00 movq $0x6,0xb78(%rax)
0055 2da5: 48 89 90 68 0b 00 00 mov %rdx,0xb68(%rax)
005c 2dac: 48 c7 80 70 0b 00 00 0e 00 00 00 movq $0xe,0xb70(%rax)
0067 2db7: bf 0b 00 00 00 mov $0xb,%edi
006c 2dbc: be 01 00 00 00 mov $0x1,%esi
0071 2dc1: e8 00 00 00 00 call 2dc6 <write_ok_or_segv+0x76> 2dc2: R_X86_64_PLT32 force_sig_fault-0x4
0076 2dc6: 4d 39 f4 cmp %r14,%r12
0079 2dc9: 0f 93 c1 setae %cl
007c 2dcc: 49 39 df cmp %rbx,%r15
007f 2dcf: 0f 93 c0 setae %al
0082 2dd2: 20 c8 and %cl,%al
0084 2dd4: 5b pop %rbx
0085 2dd5: 41 5c pop %r12
0087 2dd7: 41 5e pop %r14
0089 2dd9: 41 5f pop %r15
008b 2ddb: e9 00 00 00 00 jmp 2de0 <__pfx_get_gate_vma> 2ddc: R_X86_64_PLT32 __x86_return_thunk-0x4
Well, it got the untag right, but OMG.. :-( Joao, Sami, any idea why it
put an ENDBR in there?
