Re: [patch] i386, vdso=[0|1] boot option and /proc/sys/vm/vdso_enabled

[Andrew Morton <akpm@osdl.org>]

Rusty Russell <rusty@rustcorp.com.au> wrote:
>
> Hypervisors want to use memory at the top of the address space
>  (eg. 64MB for Xen, or 168MB for Xen w/ PAE).  Creating this hole means
>  moving the vsyscall page away from 0xffffe000.
> 
>  If we create this hole statically with a config option, we give up,
>  say, 256MB of lowmem for the case where a hypervisor-capable kernel is
>  actually running on native hardware.
> 
>  If we create this hole dynamically and leave the vsyscall page at the
>  top of kernel memory, we would have to patch up the vsyscall elf
>  header at boot time to reflect where we put it.
> 
>  Instead, this patch moves the vsyscall page into the user address
>  region, just below PAGE_OFFSET: it's still at a fixed address, but
>  it's not where the hypervisor wants to be, so resizing the hole is
>  trivial.

Seems to work.
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