Re: [patch] paravirt: VDSO page is essential

[Andi Kleen]

> VDSO is only a problem if (1) the hypervisor wants to reserve the top
> virtual address space (CONFIG_PARAVIRT=y), and (2) the glibc is old and

It broke the boot even with native hardware, no hypervisor

>  
> +config RESERVE_TOP
> +	bool
> +	help
> +	  Many hypervisors want to reserve some amount of the top of
> +	  virtual address space.  Unfortunately, old glibc needs the
> +	  vdso page there, so we must disable vdso if COMPAT_VDSO is
> +	  enabled as well as this option.

But this still means I would need to decide between a PARAVIRT
kernel that either supports xen/VMI or cannot boot old user land without
weird options.  I don't think that's the correct solution. The goal
is a single binary that runs everywhere and is still compatible.

What would probably work is to somehow decide at runtime if a hypervisor
is there or not and then set vdso default based on that. I guess that
detection would be hypervisor specific though and probably would
need paravirt ops extensions.

-Andi

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