Re: [PATCH 03/25] xen: Add nosegneg capability to the vsyscall page notes

[Jeremy Fitzhardinge <jeremy@goop.org>]

Roland McGrath wrote:
>> I have to admit I still don't really understand all this.  Is it
>> documented somewhere?
>>     
>
> I have explained it in public more than once, but I don't know off hand
> anywhere that was helpfully recorded.
>   

Thanks very much.  I'd been poking about, but the closest I came to an
actual description was various patches fixing bugs, so it was a little
incomplete.

> For example, a Xen-enabled kernel can use a single vDSO image (or a single
> pair of int80/sysenter images), containing the "nosegneg" hwcap note.  When
> there is no need for it (native or hvm or 64-bit hv or whatever), it just
> clears the mask word.  If you actually do this, you'll want to modify the
> NOTE_KERNELCAP_BEGIN macro to define a global label you can use with VDSO_SYM.
>   

Thanks for the pointer.  I'd been getting a bit of heat for enabling the
nonegseg flag unconditionally.  If I can make Xen-specific then that
will be one less source of complaints.

    J
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