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

[Tim Deegan <Tim.Deegan@xxxxxxxxxx>]

Hi,

At 14:51 +0000 on 18 Feb (1234968681), Jiang, Yunhong wrote: 
> Can you please share me which existing PET-updating code? I browsed
> the code and didn't find approprate function, especially considering
> we need update all level page tables.

xen/arch/x86/mm.c: mod_l1_entry() mod_l2_entry(), etc.

> Just as you stated before, there may have some corner case need
> considered, grant table etc (some is missed in my previous patch). For
> example, if guest has been granted, but the remote domain has not map
> it (i.e. it is in grant_table->shared, but not been mapped still),
> there should have no reference added, but if we don't hold the grant
> table lock, then the backend may mapped a wrong mfn. Such situation is
> difficult to be solved in tools. BTW, I suspect I may missed more
> reference count, for example, the page may be pinned etc, I will
> consider that in my next patch.

It should be possible to have the tools do all the PTE manipulations
with MMU update hypercalls (I think -- Keir may correct me here). Then 
the final hypercall to surrender the page will fail if the grant tables
are wrong; if it does, put the PTEs back and fall back to a live
migration.  Isn't that what your in-xen code does anyway?

> Also, as to your suggestion in previous mail of "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"

As you already pointed out, I was wrong about that -- you need to
suspend the guest to do the p2m manipulations. 

Cheers,

Tim.

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

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