RE: [Xen-devel] [PATCH] reenable pygrub build

[Rik van Riel <riel@xxxxxxxxxx>]

On Sat, 8 Oct 2005, Ian Pratt wrote:

> BTW: Have you guys got other patches to feed upstream? E.g. the stuff to 
> make your xen-friendly glibc selection work?

This is what we use; ld-linux parses the vsyscall page ELF
notes and uses the "nosegneg" glibc variant to ensure that
things work fine.

Signed-off-by: Rik van Riel <riel@xxxxxxxxxx>
Signed-off-by: Roland McGrath <roland@xxxxxxxxxx>

--- linux-2.6/arch/xen/i386/kernel/vsyscall-note.S
+++ linux-2.6/arch/xen/i386/kernel/vsyscall-note.S
@@ -0,0 +1,32 @@
+/*
+ * This supplies .note.* sections to go into the PT_NOTE inside the vDSO text.
+ * Here we can supply some information useful to userland.
+ * First we get the vanilla i386 note that supplies the kernel version info.
+ */
+
+#include "../../../i386/kernel/vsyscall-note.S"
+
+/*
+ * Now we add a special note telling glibc's dynamic linker a fake hardware
+ * flavor that it will use to choose the search path for libraries in the
+ * same way it uses real hardware capabilities like "mmx".
+ * We supply "nosegneg" as the fake capability, to indicate that we
+ * do not like negative offsets in instructions using segment overrides,
+ * since we implement those inefficiently.  This makes it possible to
+ * install libraries optimized to avoid those access patterns in someplace
+ * like /lib/i686/tls/nosegneg.  Note that an /etc/ld.so.conf.d/file
+ * corresponding to the bits here is needed to make ldconfig work right.
+ * It should contain:
+ *     hwcap 0 nosegneg
+ * to match the mapping of bit to name that we give here.
+ */
+#define NOTE_KERNELCAP_BEGIN(ncaps, mask) \
+       ASM_ELF_NOTE_BEGIN(".note.kernelcap", "a", "GNU", 2) \
+       .long ncaps, mask
+#define NOTE_KERNELCAP(bit, name) \
+       .byte bit; .asciz name
+#define NOTE_KERNELCAP_END ASM_ELF_NOTE_END
+
+NOTE_KERNELCAP_BEGIN(1, 1)
+NOTE_KERNELCAP(1, "nosegneg")  /* Change 1 back to 0 when glibc is fixed! */
+NOTE_KERNELCAP_END

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