linux-kernel - Re: [PATCH v4 07/10] xen: introduce XENMEM_exchange_and_pin and XENMEM

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Message-ID: <alpine.DEB.2.02.1308291646130.6397@kaball.uk.xensource.com>
Date:	Thu, 29 Aug 2013 16:54:21 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	Stefano Stabellini <stefano.stabellini@...citrix.com>,
	<xen-devel@...ts.xensource.com>, <linux-kernel@...r.kernel.org>,
	<linux-arm-kernel@...ts.infradead.org>, <Ian.Campbell@...rix.com>
Subject: Re: [PATCH v4 07/10] xen: introduce XENMEM_exchange_and_pin and
 XENMEM_unpin

On Thu, 15 Aug 2013, Konrad Rzeszutek Wilk wrote:
> On Thu, Aug 15, 2013 at 12:10:51PM +0100, Stefano Stabellini wrote:
> > XENMEM_exchange can't be used by autotranslate guests because of two
> > severe limitations:
> > 
> > - it does not copy back the mfns into the out field for autotranslate
> >   guests;
> > 
> > - it does not guarantee that the hypervisor won't change the p2m
> >   mappings for the exchanged pages while the guest is using them. Xen
> >   never promises to keep the p2m mapping stable for autotranslate guests
> >   in general.  In practice it won't happen unless one uses uncommon
> >   features like memory sharing or paging.
> > 
> > To overcome these problems I am introducing two new hypercalls.
> > 
> > Signed-off-by: Stefano Stabellini <stefano.stabellini@...citrix.com>
> > 
> > 
> > Changes in v4:
> > - rename XENMEM_get_dma_buf to XENMEM_exchange_and_pin;
> > - rename XENMEM_put_dma_buf to XENMEM_unpin;
> > - improve the documentation of the new hypercalls;
> > - add a note about out.address_bits for XENMEM_exchange.
> > ---
> >  include/xen/interface/memory.h |   37 +++++++++++++++++++++++++++++++++++++
> >  1 files changed, 37 insertions(+), 0 deletions(-)
> > 
> > diff --git a/include/xen/interface/memory.h b/include/xen/interface/memory.h
> > index 2ecfe4f..5708ff8 100644
> > --- a/include/xen/interface/memory.h
> > +++ b/include/xen/interface/memory.h
> > @@ -66,6 +66,7 @@ struct xen_memory_exchange {
> >      /*
> >       * [IN] Details of memory extents to be exchanged (GMFN bases).
> >       * Note that @in.address_bits is ignored and unused.
> > +     * @out.address_bits should contain the address mask for the new pages.
> >       */
> >      struct xen_memory_reservation in;
> >  
> > @@ -263,4 +264,40 @@ struct xen_remove_from_physmap {
> >  };
> >  DEFINE_GUEST_HANDLE_STRUCT(xen_remove_from_physmap);
> >  
> > +#define XENMEM_exchange_and_pin             26
> > +/*
> > + * This hypercall is similar to XENMEM_exchange: it takes the same
> > + * struct as an argument and it exchanges the pages passed in with a new
> > + * set of pages. The new pages are going to be "pinned": it's guaranteed
> > + * that their p2m mapping won't be changed until explicitly "unpinned".
> > + * The content of the exchanged pages is lost.
> > + * Only normal guest r/w memory can be pinned: no granted pages or
> > + * ballooned pages.
> > + * If return code is zero then @out.extent_list provides the DMA frame
> > + * numbers of the newly-allocated memory.
> > + * Returns zero on complete success, otherwise a negative error code:
> > + *   -ENOSYS if not implemented
> > + *   -EINVAL if the page is already pinned
> > + *   -EFAULT if the physical to machine walk fails
> 
> I don't know what that means. Physical to machine walk? That sounds like
> you are trying to do some form of mind melt between robots and humans
> while walking around the office.
> 
> Could you expand this a bit please?

Actually pfn to mfn resolution is not the only possible cause of
-EFAULT, for example all the copy_to/from_guest failures return
-EFAULT.
I am just going to go for the generic

"if an internal error occurs"
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