RE: [Xen-devel] page table question!

["Petersson, Mats" <Mats.Petersson@xxxxxxx>]

> -----Original Message-----
> From: M.A. Williamson [mailto:maw48@xxxxxxxxxxxxxxxx] On 
> Behalf Of Mark Williamson
> Sent: 13 June 2007 17:06
> To: xen-devel@xxxxxxxxxxxxxxxxxxx
> Cc: Petersson, Mats; MT Rezaie
> Subject: Re: [Xen-devel] page table question!
> 
> > > I want to know any difference between writing, reading 
> and shadow page
> > > table (for example performance).
> >
> > Not sure exactly what you're asking for: are you asking 
> about "writable"
> > and "non-writable" page-table vs. shadow-page-table?
> 
> "Writable pagetables" is an interface introduced for Xen 3 PV 
> guests to update 
> their pagetables.  Previous versions of Xen required guests 
> to be modified to 
> make every update to their pagetables explicitly call into Xen (and 
> explicitly batch those operations).  Writeable pagetables 
> replace this by 
> allowing updates to the pagetables to be implemented by the 
> guest as attempts 
> to write directly to them.  This isn't a trusted operation, 
> however, since 
> Xen uses page protections to prevent unvalidated writes going 
> through...  
> Right now this is implemented by trapping each attempt to 
> write to the lower 
> levels of the pagetables and emulating it in Xen.  Previous 
> implementations 
> were a bit more complicated but found to be unnecessary.

Thanks for clarifying. I seem to remember reading about a mode where the
page-table isn't "read-only" - did I just imagine that, or what?
> 
> This interface does require the guest to do physical->machine 
> translations 
> (which are verified by Xen before installing them), and the 
> guest actually 
> works with the real pagetables (subject to certain 
> restrictions).  As a 
> result the guest can get data out of the real pagetables by 
> doing a simple 
> read - this operation should be as fast as native.  Updating 
> pagetables is 
> obviously a bit slower than native because Xen has to emulate 
> them; explicit 
> calls to Xen's batched update interface are used to give 
> decent performance 
> when lots of PTEs are being modified.
> 
> Does that answer your question, Mohammad?
> 
> > As of currently, Shadow-mode is only used for HVM (fully 
> virtualized)
> > domains. In this case, the guest has a "dummy" page-table 
> that is not
> > used by the processor for any memory accesses, instead a 
> "shadow" (copy)
> > of the guest's page-table is used. The shadow page-table is 
> used by the
> > processor. To make updates to the page-table appear in the
> > shadow-page-table, the hypervisor is making sure that the
> > guest-page-table is read-only, and when the guest writes to the
> > page-table, we reproduce the write [with modification to reflect the
> > difference between guest physical and machine physical address].
> 
> What Mats said :-)

There is another mode that we haven't mentioned yet, which is the
HAP-mode (aka NPT (Nested Page-tables) from AMD, and Intel has something
similar which I believe is called EPT). HAP stands for Hardware Assisted
Paging. This mode is (obviously) only available in processors that have
support for this feature - there are none of these available on the
market just yet.

The idea in this mode is that the guest doesn't know about the
machine-physical address, but we also don't intercept page-table writes
(by making the guest page-table read-only). Instead, there is a second,
hypervisor-owned, page-table that translates the guest-physical address
to machine-physical address. 

So instead of using a single page-table that is "modified" to match the
guest-virtual->machine-physical, we add another layer of page-table to
translate guest-physical->machine-physical address. This second
page-table is normally static, so it requires no changes under normal
operation. 

There is a performance advantage here in that we don't have to intercept
the write to the page-table. This is normally fairly expensive, so for
systems where page-tables are written often, it's definitely worth
having. On systems with rare updates to the page-table, the performance
advantage is much smaller, if any at all. 

> 
> One thing I've never been clear on for shadow mode is how 
> accessed / dirty 
> bits get propagated to the guest pagetable from the shadow.

Good question. I have a feeling that the answer is "it doesn't". HAP
would probably solve this problem. 

--
Mats

> 
> At some point there was also support in Xen for fully 
> translated shadowing of 
> PV guests but I'm not sure this works now.  PV guests do 
> still get shadowed 
> in a limited way during live migration, however.
> 
> Cheers,
> Mark
> 
> -- 
> Dave: Just a question. What use is a unicyle with no seat?  
> And no pedals!
> Mark: To answer a question with a question: What use is a skateboard?
> Dave: Skateboards have wheels.
> Mark: My wheel has a wheel!
> 
> 
> 



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