[Xen-devel] Re: [RFC][PATCH] Basic support for page offline

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

At 04:48 -0500 on 15 Feb (1234673293), Jiang, Yunhong wrote:
> > The reference counting in update_pgtable_entry() is confusing -- it
> > should probably always do reference counting for both the old and new
> > entries; that seems more robust than only doing the decrements
> > there and
> > manually setting count_info and type_info on the new page in replace_page.
> 
> Sure, I will do like this.

In fact, it should use the existing PTE-updating code -- I suspect that,
for example, your code won't work at all on a guest that has shadow
pagetables enabled.

> > The tools patch is enormous, and seems to copy big chunks of
> > xc_domain_save into a new file.  And since Xen is now doing the hard
> > work of pagetable manipulation, I don't think you even need to suspend
> > the guest -- just pausing it should be enough and is much easier.
> 
> But I'm not sure if we can update the P2M table from Xen side, that's
> the reason I did the it in the user space.

In that case, why don't you update the pagetables from the tools as
well?  That way you'd avoid walking the guest pagetables in Xen.  You
could make all the PTE changes, try to free the page, and if it still
doesn't work (because there's some other refcount held), put things back
the way they were.

Tim.

-- 
Tim Deegan <Tim.Deegan@xxxxxxxxxx>
Principal Software Engineer, Citrix Systems (R&D) Ltd.
[Company #02300071, SL9 0DZ, UK.]

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