Re: [Xen-devel] [PVH]: Help: msi.c

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.12.12 at 13:19, Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
wrote:
> On Thu, 13 Dec 2012, Jan Beulich wrote:
>> >>> On 13.12.12 at 02:43, Mukesh Rathor <mukesh.rathor@xxxxxxxxxx> wrote:
>> > On Wed, 12 Dec 2012 17:15:23 -0800
>> > Mukesh Rathor <mukesh.rathor@xxxxxxxxxx> wrote:
>> > 
>> >> On Tue, 11 Dec 2012 12:10:19 +0000
>> >> Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx> wrote:
>> >> 
>> >> > On Tue, 11 Dec 2012, Mukesh Rathor wrote:
>> >> > > On Mon, 10 Dec 2012 09:43:34 +0000
>> >> > > "Jan Beulich" <JBeulich@xxxxxxxx> wrote:
>> >> > 
>> >> > That's strange because AFAIK Linux is never editing the MSI-X
>> >> > entries directly: give a look at
>> >> > arch/x86/pci/xen.c:xen_initdom_setup_msi_irqs, Linux only remaps
>> >> > MSIs into pirqs using hypercalls. Xen should be the only one to
>> >> > touch the real MSI-X table.
>> >> 
>> >> So, this is what's happening. The side effect of :
>> >> 
>> >>         if ( rangeset_add_range(mmio_ro_ranges, dev->msix_table.first,
>> >>                                 dev->msix_table.last) )
>> >>             WARN();
>> >>         if ( rangeset_add_range(mmio_ro_ranges, dev->msix_pba.first,
>> >>                                 dev->msix_pba.last) )
>> >>             WARN();
>> >> 
>> >> in msix_capability_init() in xen is that the dom0 EPT entries that
>> >> I've mapped are going from RW to read only. Then when dom0 accesses
>> >> it, I get EPT violation. In case of pure PV, the PTE entry to access
>> >> the iomem is RW, and the above rangeset adding doesn't affect it. I
>> >> don't understand why? Looking into that now...
>> 
>> As far as I was able to tell back at the time when I implemented
>> this, existing code shouldn't have mappings for these tables in
>> place at the time these ranges get added here. But I noted in
>> the patch description that this is a potential issue (and may need
>> fixing if deemed severe enough - back then, apparently nobody
>> really cared, perhaps largely because passthrough to PV guests
>> isn't considered fully secure anyway).
>> 
>> Now - did that change? I.e. can you describe where the mappings
>> come from that cause the problem here?
> 
> The generic Linux MSI-X handling code does that, before calling the
> arch specific msi setup function, for which we have a xen version
> (xen_initdom_setup_msi_irqs):
> 
> pci_enable_msix -> msix_capability_init -> msix_map_region

Ah, okay, (of course?) I had looked only at the forward ported
version of this. Is all that fiddling with the mask bits really
being suppressed properly when running under Xen? Otherwise
pv-ops is quite broken in this regard at present... And if it is,
I don't see what the respective ioremap() is good for here in
the Xen case.

Jan


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