Re: [Xen-devel] [v1 1/2] xen/p2m: Create identity mappings for PFNs beyound E820 and PCI BARs

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Oct 25, 2013 at 04:08:19PM -0600, Bjorn Helgaas wrote:
> On Fri, Oct 25, 2013 at 9:03 AM, Konrad Rzeszutek Wilk
> <konrad.wilk@xxxxxxxxxx> wrote:
> > On bootup the E820 "gaps" or E820_RESV regions are marked as
> > identity regions. Meaning that any lookup done in the P2M
> > will return the same value: pfn_to_mfn(pfn) == pfn.
> >
> > This is needed for PCI devices so that drivers can reference
> > the correct bus address. Unfortunatly there are also PCIe
> > devices which setup their MMIO region above the E820. By default
> > we assume in the P2M that any region above the last E820 entry
> > is to be used for ballooning. That is not true - and we don't
> > mark such regions as IDENTIY_FRAME but INVALID_P2M_ENTRY.
> > The result is that any lookup in the P2M (pfn_to_mfn(pfn) == 0)
> > gets us the 0 bus address which is hardly correct.
> >
> > A solution that this patch implements is to walk the PCI device
> > BAR regions and mark them as IDENTITY_FRAMEs in the P2M.
> > Naturally some checks are needed such as making sure that the
> > BAR regions are not part of the balloon pages, nor regular RAM.
> >
> > We only set them to IDENTITY if the:
> >  pfn_to_mfn(pfn) == INVALID_P2M_ENTRY.
> >
> > Another solution would be to mark all P2M entries beyond the
> > last E820 entry _and_ not in the balloon regions as IDENTITY.
> >
> > If done, that means in worst case we have to reserve MAX_DOMAIN_PAGES
> > pages (so 2MB) of virtual space in case we have to create
> > new P2M leafs. We could be fancy and make the P2M code understand
> > p2m_mid_missing and p2_mid_identity and do the right things.
> > But that is quite complex while this particular patch only
> > gets invoked if there are PCI devices. Another solution (David
> > Vrabel ideas) was to consider INVALID_P2M_ENTRY as 1-1 regions.
> > The author of this patch is not sure of the ramifications
> > as it would require surgery in various P2M codebits.
> >
> > Reported-by: Lukas Hejtmanek <xhejtman@xxxxxxxxxxx>
> > Reported-by: Lance Larsh <lance.larsh@xxxxxxxxxx>
> > CC: Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>
> > CC: David Vrabel <david.vrabel@xxxxxxxxxx>
> > Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
> > ---
> >  drivers/xen/balloon.c | 19 +++++++++++++
> >  drivers/xen/pci.c     | 79 
> > +++++++++++++++++++++++++++++++++++++++++++++++++--
> >  include/xen/balloon.h |  1 +
> >  3 files changed, 96 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/xen/balloon.c b/drivers/xen/balloon.c
> > index b232908..258e3f9 100644
> > --- a/drivers/xen/balloon.c
> > +++ b/drivers/xen/balloon.c
> > @@ -133,6 +133,25 @@ static void balloon_append(struct page *page)
> >         adjust_managed_page_count(page, -1);
> >  }
> >
> > +/*
> > + * Check if any the balloon pages overlap with the supplied
> > + * pfn and its range.
> > + */
> > +bool balloon_pfn(unsigned long pfn, unsigned long nr)
> > +{
> > +       struct page *page;
> > +
> > +       if (list_empty(&ballooned_pages))
> > +               return false;
> > +
> > +       list_for_each_entry(page, &ballooned_pages, lru) {
> > +               unsigned long b_pfn = page_to_pfn(page);
> > +
> > +               if (b_pfn >= pfn && b_pfn < pfn + nr)
> > +                       return true;
> > +       }
> > +       return false;
> > +}
> >  /* balloon_retrieve: rescue a page from the balloon, if it is not empty. */
> >  static struct page *balloon_retrieve(bool prefer_highmem)
> >  {
> > diff --git a/drivers/xen/pci.c b/drivers/xen/pci.c
> > index 18fff88..6b86eda 100644
> > --- a/drivers/xen/pci.c
> > +++ b/drivers/xen/pci.c
> > @@ -22,6 +22,9 @@
> >  #include <xen/xen.h>
> >  #include <xen/interface/physdev.h>
> >  #include <xen/interface/xen.h>
> > +#include <xen/interface/memory.h>
> > +#include <xen/page.h>
> > +#include <xen/balloon.h>
> >
> >  #include <asm/xen/hypervisor.h>
> >  #include <asm/xen/hypercall.h>
> > @@ -127,6 +130,72 @@ static int xen_add_device(struct device *dev)
> >         return r;
> >  }
> >
> > +static void xen_p2m_add_device(struct device *dev)
> > +{
> > +       int i;
> > +       struct pci_dev *pci_dev = to_pci_dev(dev);
> > +
> > +       /* Verify whether the MMIO BARs are 1-1 in the P2M. */
> > +       for (i = 0; i < PCI_NUM_RESOURCES; i++) {
> 
> Kudos for avoiding the usual "for_each_pci_dev()" trap (which totally
> ignores hot-added devices).

Thank you :-)
> 
> But this still seems a bit fragile because:
> 
> (1) I would guess the same consideration applies to all PCI MMIO
> space, regardless of whether it is actually assigned to a BAR.  So
> maybe you want to do the same thing for currently-unassigned MMIO
> space.

OK. Let me dig in the code to find where that is..
> 
> (2) It will break if PCI ever reassigns device resources.  We
> currently don't reassign anything after we finish the initial
> enumeration and configuration of the device, but it's conceivable that
> we could someday.

<nods>
> 
> If you can look at PCI host bridge apertures instead of BARs, that
> would solve both problems.  Reassigning those apertures is
> theoretically possible but is not even a gleam in our eyes yet.

<nods> I think I have to have both (BARs and host bridge apertures) as when
we do PCI passthrough to a guest - we might do it without a bridge.


> 
> > +               unsigned long pfn, start, end, ok_pfns;
> > +               char bus_addr[64];
> > +               char *fmt;
> > +
> > +               if (!pci_resource_len(pci_dev, i))
> > +                       continue;
> > +
> > +               if (pci_resource_flags(pci_dev, i) == IORESOURCE_IO)
> 
> You probably want:
> 
>     if (pci_resource_flags(pci_dev, i) & IORESOURCE_IO)
> 
> here.  And I don't understand what you're doing below, but my
> impression is you only care about MEM regions, not IO regions, so it
> seems like you might want to skip IO completely.

<nods> Good catch!

However there are is one thing this patch does not cover - that is if the
PCI bridge window is outside the P2M tree we keep (the P2M tree is a lookup
system to construct the PTE's with valid and correct physical addresses) we
end up always returning 0. David Vrabel suggests to return just the PFN value
which would certainly solve this particular problem - but I am bit
uncomfortable of doing surgery in that code.

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