Re: bad paravirt/Xen interaction in "x86 - Enhance DEBUG_RODATA support- alternatives"

[Jeremy Fitzhardinge]

H. Peter Anvin wrote:
> Jeremy Fitzhardinge wrote:
> > The patch "x86 - Enhance DEBUG_RODATA support - alternatives" enables 
> > the kernel for writing by clearing X86_CR0_WP allow privileged 
> > writes.  This won't work in a paravirt environment for two reasons:
> >
> >   1. the kernel may not be running in ring 0, so writes will still be
> >      prevented
> >   2. the hypervisor prevents X86_CR0_WP from being cleared anyway (it
> >      GPFs the cr0 update)
> >
> > This crashes on Xen, and it would probably break VMI too.

(lguest too, of course)

> > The only safe way to allow writes is to change the page permissions 
> > (either on the page itself, or create a temporary writable alias for 
> > that page).  Perhaps something you could do it with kmap_atomic.
>
> A properly implemented hypervisor should arguably emulate this.
>
> Doesn't really mean the patch is worth the pain.

No, it would be irritating to implement.

Seems to me that doing the update in a temporary kmap_atomic mapping 
would be a more straightforward way to go, anyway.  How would you 
implement this on a processor without something like X86_CR0_WP?

   J
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