24037a8b69
Add the "nosegneg" fake capabilty to the vsyscall page notes. This is used by the runtime linker to select a glibc version which then disables negative-offset accesses to the thread-local segment via %gs. These accesses require emulation in Xen (because segments are truncated to protect the hypervisor address space) and avoiding them provides a measurable performance boost. Signed-off-by: Ian Pratt <ian.pratt@xensource.com> Signed-off-by: Christian Limpach <Christian.Limpach@cl.cam.ac.uk> Signed-off-by: Chris Wright <chrisw@sous-sol.org> Signed-off-by: Jeremy Fitzhardinge <jeremy@xensource.com> Acked-by: Zachary Amsden <zach@vmware.com> Cc: Roland McGrath <roland@redhat.com> Cc: Ulrich Drepper <drepper@redhat.com>
43 lines
1.5 KiB
ArmAsm
43 lines
1.5 KiB
ArmAsm
/*
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* This supplies .note.* sections to go into the PT_NOTE inside the vDSO text.
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* Here we can supply some information useful to userland.
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*/
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#include <linux/version.h>
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#include <linux/elfnote.h>
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/* Ideally this would use UTS_NAME, but using a quoted string here
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doesn't work. Remember to change this when changing the
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kernel's name. */
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ELFNOTE_START(Linux, 0, "a")
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.long LINUX_VERSION_CODE
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ELFNOTE_END
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#ifdef CONFIG_XEN
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/*
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* Add a special note telling glibc's dynamic linker a fake hardware
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* flavor that it will use to choose the search path for libraries in the
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* same way it uses real hardware capabilities like "mmx".
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* We supply "nosegneg" as the fake capability, to indicate that we
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* do not like negative offsets in instructions using segment overrides,
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* since we implement those inefficiently. This makes it possible to
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* install libraries optimized to avoid those access patterns in someplace
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* like /lib/i686/tls/nosegneg. Note that an /etc/ld.so.conf.d/file
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* corresponding to the bits here is needed to make ldconfig work right.
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* It should contain:
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* hwcap 1 nosegneg
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* to match the mapping of bit to name that we give here.
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*/
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/* Bit used for the pseudo-hwcap for non-negative segments. We use
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bit 1 to avoid bugs in some versions of glibc when bit 0 is
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used; the choice is otherwise arbitrary. */
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#define VDSO_NOTE_NONEGSEG_BIT 1
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ELFNOTE_START(GNU, 2, "a")
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.long 1, 1<<VDSO_NOTE_NONEGSEG_BIT /* ncaps, mask */
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.byte VDSO_NOTE_NONEGSEG_BIT; .asciz "nosegneg" /* bit, name */
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ELFNOTE_END
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#endif
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