linux-kernel - Re: [patch 6/6] Guest page hinting: s390 support.

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Date:	Thu, 13 Mar 2008 18:23:38 +0100
From:	Martin Schwidefsky <schwidefsky@...ibm.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	akpm@...l.org, linux-s390@...r.kernel.org, frankeh@...son.ibm.com,
	nickpiggin@...oo.com.au, linux-kernel@...r.kernel.org,
	virtualization@...ts.osdl.org,
	Anthony Liguori <anthony@...emonkey.ws>, hugh@...itas.com
Subject: Re: [patch 6/6] Guest page hinting: s390 support.

On Thu, 2008-03-13 at 10:05 -0700, Jeremy Fitzhardinge wrote:
> Martin Schwidefsky wrote:
> > Vz is the page discarded state. The difference to Uz is slim, both
> > states will cause a program check on access. Vz generates a discard
> > fault, Uz generates an addressing exception which is nice for debugging.
> >   
> 
> How do you handle these different cases in Linux?  Do you use Vr pages 
> in the pagecache, and then shoot down the pagecache entry if the host 
> steals the page?

The environment where we currently run all this is z/VM as the host and
Linux as the guest. We have two page tables on s390, a host page table
and a guest page table. If the host discards a page it simple removes
the entry for the page in the host page table. If the guest comes along
and accesses the page the host gets the fault and generates the
appropriate fault.

> The Uz access exception presumably just generates a normal oops.

Yes, the handler for an addressing exception will call die() for a
kernel check without a fixup.

> (I should probably make time to read the rest of the series.)
> 
> >> But given how you've described V-state pages, I really would expect 
> >> writes to a Vz to work, or alternatively, all writes to V-state pages to 
> >> be disallowed.  Are there any real uses for a writable Vr page?
> >>     
> >
> > You mean in the section that speaks about the guests states S/U/V/P ?
> > Always keep in mind that you can access a V/P page only until it gets
> > discarded. Then the useful content of the page frame is lost and any
> > read of write to the not Vz page will be answered with a discard fault.
> >   
> 
> Presumably reads from a Vz page also generate a discard fault?

Yes.

> > A Vr page is read-only. If a page gets mapped for writing it needs to
> > get into the Pr state. This is the hint for the host to look at the
> > dirty bit before it discards a page.
> > So yes, there is no use for a writable Vr page.
> >   
> 
> OK, thanks, that clears things up.  I was assuming that Vr was 
> technically writable but that writes could be discarded at any time (ie, 
> allowing guests to merrily shoot themselves in the foot ;).  Making it 
> forced RO is much more sensible.

Well, technically you could write to a Vr page via the kernel address
space. The thing is that the host can just discard the page although it
is dirty. The Vr state is used for page cache pages which do not have
any writable mapping.

-- 
blue skies,
  Martin.

"Reality continues to ruin my life." - Calvin.


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