[Xen-devel] Re: [patch 17/26] Xen-paravirt_ops: Add nosegneg capability to the vsyscall page notes

[Ingo Molnar <mingo@xxxxxxx>]

i've Cc:-ed Roland and Ulrich, who should make the call on this one.

* Jeremy Fitzhardinge <jeremy@xxxxxxxx> wrote:

> 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@xxxxxxxxxxxxx>
> Signed-off-by: Christian Limpach <Christian.Limpach@xxxxxxxxxxxx>
> Signed-off-by: Chris Wright <chrisw@xxxxxxxxxxxx>
> Acked-by: Zachary Amsden <zach@xxxxxxxxxx>
> 
> ---
>  arch/i386/kernel/vsyscall-note.S |   28 ++++++++++++++++++++++++++++
>  1 file changed, 28 insertions(+)
> 
> ===================================================================
> --- a/arch/i386/kernel/vsyscall-note.S
> +++ b/arch/i386/kernel/vsyscall-note.S
> @@ -23,3 +24,31 @@ 3: .balign 4;              /* pad out section */           
>          
>       ASM_ELF_NOTE_BEGIN(".note.kernel-version", "a", UTS_SYSNAME, 0)
>       .long LINUX_VERSION_CODE
>       ASM_ELF_NOTE_END
> +
> +#ifdef CONFIG_XEN
> +/*
> + * 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")
> +NOTE_KERNELCAP_END
> +#endif
> +
> 
> -- 
> 
> -
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