Re: [Xen-devel] [PATCH v5 3/4] xen: implement guest_physmap_pin_range and guest_physmap_unpin_range

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2013-09-10 at 20:11 +0100, Stefano Stabellini wrote:
> On Tue, 10 Sep 2013, Ian Campbell wrote:
> > On Mon, 2013-09-09 at 17:06 +0100, Stefano Stabellini wrote:
> > > guest_physmap_pin_range pins a range of guest pages so that their p2m
> > > mappings won't be changed.
> > > guest_physmap_unpin_range unpins the previously pinned pages.
> > > The pinning is done using one of the spare bits in the p2m ptes.
> > > 
> > > Use the newly introduce p2m_walker to implement the two functions on
> > > ARM.
> > > Provide empty stubs for x86.
> > > 
> > > 
> > > Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> > > 
> > > 
> > > Changes in v5:
> > > - return -EBUSY when the P2M_DMA_PIN check fails;
> > > - rename _guest_physmap_pin_range to pin_one_pte;
> > > - rename _guest_physmap_unpin_range to unpin_one_pte.
> > > 
> > > Changes in v4:
> > > - use p2m_walker to implement guest_physmap_pin_range and
> > > guest_physmap_unpin_range;
> > > - return -EINVAL when the P2M_DMA_PIN check fails;
> > > - change the printk into a gdprintk;
> > > - add a comment on what type of page can be pinned.
> > > ---
> > >  xen/arch/arm/p2m.c         |   48 
> > > ++++++++++++++++++++++++++++++++++++++++++++
> > >  xen/include/asm-arm/mm.h   |    4 +++
> > >  xen/include/asm-arm/page.h |    7 +++++-
> > >  xen/include/asm-x86/p2m.h  |   12 +++++++++++
> > >  4 files changed, 70 insertions(+), 1 deletions(-)
> > > 
> > > diff --git a/xen/arch/arm/p2m.c b/xen/arch/arm/p2m.c
> > > index a9ceacf..bac6c7e 100644
> > > --- a/xen/arch/arm/p2m.c
> > > +++ b/xen/arch/arm/p2m.c
> > > @@ -154,6 +154,46 @@ paddr_t p2m_lookup(struct domain *d, paddr_t paddr)
> > >      return p2m.maddr;
> > >  }
> > >  
> > > +static int pin_one_pte(lpae_t *ptep, void *arg, int level)
> > > +{
> > > +    lpae_t pte = *ptep;
> > > +    ASSERT(level == 3);
> > > +
> > > +    if ( pte.p2m.avail & P2M_DMA_PIN )
> > > +        return -EBUSY;
> > > +    pte.p2m.avail |= P2M_DMA_PIN;
> > > +    write_pte(ptep, pte);
> > > +    return 0;
> > > +}
> > > +
> > > +int guest_physmap_pin_range(struct domain *d,
> > > +                            xen_pfn_t gpfn,
> > > +                            unsigned int order)
> > > +{
> > > +    return p2m_walker(d, gpfn << PAGE_SHIFT, order,
> > > +                      pin_one_pte, NULL);
> > 
> > If this fails then you will have left some subset of the pages
> > successfully pinned. You need to clean it up I think, or be fatal to the
> > guest or something.
> 
> Making the failure of guest_physmap_pin_range fatal to the guest would
> also solve the non-trivial problem of error handling in memory_exchange.

Actually, I don't think this will work.

Consider two concurrent I/O operations dispatched by two threads in the
same process, to/from non-overlapping regions of the same page.

Hrm, we may actually need a reference count on the pin :-(

We might need to rethink here -- i.e. use a type count on the underlying
machine page rather than a pin in the p2m entry? That kind of makes
sense since it will be the underlying page which is exposed to the
hardware...

Ian.


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