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RE: [Xen-devel] [RFC] VMI for Xen?

  • To: xen-devel@xxxxxxxxxxxxxxxxxxx
  • From: Daniel Arai <arai@xxxxxxxxxx>
  • Date: Thu, 16 Mar 2006 16:09:28 -0800
  • Delivery-date: Fri, 17 Mar 2006 00:10:36 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>

Ian Pratt wrote:

The current VMI patchset couldn't support Xen's direct-mode (non shadow)
MMU virtualization, and hasn't really been thought through properly for
SMP. I'm sure we'll get something worked out that keeps everyone happy.

I'm not sure what you mean that VMI "hasn't really been thought through
properly for SMP."  I'm writing this email from an SMP VMI guest.

Using page table shadows makes an SMP implementation really easy.  Page
table writes can go directly to the virtual page tables.  The guest needs
to inform the hypervisor which page table entries have been modified, but
actually processing these entries and updating the hardware page tables
can be deferred until page invalidate, cr3 reassignment, or TLB shootdown
time.  All of the details here are hidden behind the VMI "set page table
entry" call.  The guest just uses this call and everything happens on its

We give each VCPU a private queue for notifying the hypervisor of page
table updates and other types of CPU state changes that can be deferred.
Because we use per-VCPU page table shadows, we're able to put a private
queue in each VCPU at the same linear address space.  Updates to the queue
thus are just a couple of memory writes, and allows very efficient batching
of page table writes, even for SMP.  The hypervisor and the ROM contain
all the knowledge of this interface (rather than the guest), so it would
be relatively easy to change to support shared shadow page tables, or a
different mechanism for direct-mode page tables.

We're obviously lacking some interfaces needed for direct-mode paging, but
aside from those, are there specific deficiencies for SMP?

(Apologies if quoting/replies aren't set up properly;  I wasn't previously
subscribed to xen-devel, and had this forwarded by a colleague.)


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