RE: [Xen-devel] 46% performance drop with change in glibc

["Ian Pratt" <m+Ian.Pratt@xxxxxxxxxxxx>]

> > > The crippled ABI is supported by the code in /lib/i686/nosegneg.  
> > > This code is *automatically* used if the kernel is 
> compiled to match 
> > > the glibc version.  This happens using the
> > >
> > > linux-2.6-xen-vdso-note.patch
> >
> > I believe we have this patch in the xen repo, but I haven't ever 
> > verified whether the right thing happens under FC4.
> 
> I am not seeing this patch in ./patches/linux-2.6.12 
> directory.  Is it somewhere else?

It should be in the main tree. The appended patch was included some time
ago, but I've never tested it to see whether it works since I don't
generally run bleeding edge Fedora.

Is there some piece of the patch missing?

Thanks,
Ian


--- 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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