Re: a quick question about _PAGE_GLOBAL flag in para virt mode

[John Levon <levon@movementarian.org>]

On Sun, Feb 15, 2009 at 02:26:13PM -0800, Jeremy Fitzhardinge wrote:

> >why not set global bit for user kernel pages? I guess this is because 
> >when domain switchs, all tlb entries will still be flushed, so it's 
> >not necessary for user kernel pages. Is my understanding right?
> 
> 64-bit Xen uses paging to protect the guest kernel from guest usermode, 
> and protect Xen from the guest kernel, and as a result has to do a cr3 
> reload for any user->kernel switch (which is why it has to go via Xen).  
> We don't want kernel mappings to be visible to usermode, so they have to 
> only be in tlb while we're actually running in guest kernel mode; 

And here's a real example of what happens when you don't get this right:

http://blogs.sun.com/levon/entry/xen_compatibility_with_solaris

regards
john