Re: [Xen-devel] Regarding page table management changes from Xen v1to Xen v2 (and v3)

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 26 Apr 2006, at 19:15, Anthony Liguori wrote:

> > No, these singleton updates on the Linux demand fault path are issued
> > with the update_va_mapping hypercall. This is possible because the
> > virtual address that the pte maps is known, so its possible for Xen to
> > use a linear pagetable to do the update rather than creating an
> > on-demand mapping to the domain's pagetable page. This is an important
> > optimization on 32b where Xen address space is tight.
> Just to make sure I understand, we do the linear mapping for 
> performance (so we don't have to keep remapping) but not necessarily 
> to save VA space since the linear mapping uses a fixed amount of 
> space.
> Is there any reason that we would strictly require a linear mapping 
> (other than it being convenient)?
On 64b we could use the 1:1 mapping of all physmem that Xen has. 
Obviously that is infeasible on 32b. In fact the interface (passing a 
virtual address) pretty much means we have to use a linear mapping in 
both cases.
 -- Keir


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