Re: [Xen-devel] Re: [PATCH] Xen Guest Kexec

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 26 Feb 2006, at 22:49, Ian Pratt wrote:

> > Also switching page tables seems to be not so easy.  Is it
> > possible to switch atomically to a new, completely
> > independant page table tree?
> > i.e. old tree is valid (of cource), new tree is too, but the
> > pages of the old tree are not mapped read-only in the new
> > tree (and visa versa).
> If the pagetable you're running on isn't pinned the refcounts will drop
> to zero when you switch away from it, hence its only necessary for the
> destination pagetable to be internally consistent.
We run on the old pagetables while we validate the new ones, so 
currently they need to map each other read-only. We could have a slow 
path in the pagetable switch code that switches to the idle pagetable, 
drops the old pagetable, checks the new pagetable then switches again. 
On failure it would have to crash the domain dealing with the fact that 
this VCPU no longer has a valid 'cr3'.
It's a bit of a pain, but it might simplify mm/init.c. I'm not sure by 
how much though.
 -- Keir


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