Messages in this thread | | | Subject | Re: [PATCH 26/26] x86, pkeys: Documentation | From | Paolo Bonzini <> | Date | Fri, 2 Oct 2015 14:14:00 +0200 |
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On 02/10/2015 13:58, Linus Torvalds wrote: > On Fri, Oct 2, 2015 at 7:49 AM, Paolo Bonzini <pbonzini@redhat.com> wrote: >> On 02/10/2015 00:48, Linus Torvalds wrote: >>> It's quite likely that you will find that compilers put read-only >>> constants in the text section, knowing that executable means readable. >> >> Not on x86 (because it has large immediates; RISC machines and s390 do >> put large constants in the text section). >> >> But at the very least jump tables reside in the .text seection. > > Yes, at least traditionally gcc put things like the jump tables for > switch() statements immediately next to the code. That caused lots of > pain on the P4, where the L1 I$ and D$ were exclusive. I think that > caused gcc to then put the jump tables further away, and it might be > in a separate section these days - but it might also just be > "sufficiently aligned" that the L1 cache issue isn't in play any more. > > Anyway, because of the P4 exclusive L1 I/D$ issue we can pretty much > rest easy knowing that the data accesses and text accesses should be > separated by at least one cacheline (maybe even 128 bytes - I think > the L4 used 64-byte line size, but it was sub-sections of a 128-byte > bigger line - but that might have been in the L2 only). > > But I could easily see the compiler/linker still putting them in the > same ELF segment.
You're entirely right, it puts them in .rodata actually. But .rodata is in the same segment as .text:
$ readelf --segments /bin/true ... Section to Segment mapping: Segment Sections... 00 01 .interp 02 .interp .note.ABI-tag .note.gnu.build-id .gnu.hash .dynsym .dynstr .gnu.version .gnu.version_r .rela.dyn .rela.plt .init .plt .text .fini .rodata .eh_frame_hdr .eh_frame 03 .init_array .fini_array .jcr .data.rel.ro .dynamic .got .data .bss 04 .dynamic 05 .note.ABI-tag .note.gnu.build-id 06 .eh_frame_hdr 07 08 .init_array .fini_array .jcr .data.rel.ro .dynamic .got
Paolo
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