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

Jeremy Fitzhardinge
2007-04-24T06:18:39Z

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