Re: [PATCH v2 1/4] libxl: do not attach xen-pciback to HVM domain, if stubdomain is in use

["Marek Marczykowski-Górecki" <marmarek@invisiblethingslab.com>]

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On Wed, Jan 16, 2019 at 05:47:19PM +0100, Roger Pau Monné wrote:
> On Tue, Jan 15, 2019 at 04:36:28PM +0100, Marek Marczykowski-Górecki wrote:
> > HVM domains use IOMMU and device model assistance for communicating with
> > PCI devices, xen-pcifront/pciback is used only in PV domains.
> 
> You still need pciback in order to reset the device when it's
> deassigned from the guest, so it's functionality is not only used by
> PV guests.

Right, I'll update the commit message to match v2 code.

> > When HVM domain has device model in stubdomain, attaching xen-pciback to
> > the target domain itself is not only useless, but also may prevent
> > attaching xen-pciback to the stubdomain, effectively breaking PCI
> > passthrough.
> 
> Right. When doing passthrough with a stubdomain you want the target
> domain to have the memory and IO regions mapped, and the stubdomain to
> handle the rest.
> 
> > Signed-off-by: Marek Marczykowski-Górecki <marmarek@invisiblethingslab.com>
> > ---
> > Changes in v2:
> >  - previously called "libxl: attach xen-pciback only to PV domains"
> >  - instead of excluding all HVMs, change the condition to what actually
> >    matters here - check if stubdomain is in use; this way xen-pciback is
> >    always in use (either for the target domain, or it's stubdomain),
> >    fixing PCI reset by xen-pciback concerns
> > ---
> >  tools/libxl/libxl_pci.c | 4 ++--
> >  1 file changed, 2 insertions(+), 2 deletions(-)
> > 
> > diff --git a/tools/libxl/libxl_pci.c b/tools/libxl/libxl_pci.c
> > index 87afa03..3b6b23c 100644
> > --- a/tools/libxl/libxl_pci.c
> > +++ b/tools/libxl/libxl_pci.c
> > @@ -1106,7 +1106,7 @@ out:
> >          }
> >      }
> >  
> > -    if (!starting)
> > +    if (!starting && !libxl_get_stubdom_id(CTX, domid))
> 
> This change seems to assume that both libxl_domain_config for the
> target and the stubdomain will have the assigned pci devices in the
> pcidevs field. 

Not really. libxl__device_pci_add() calls do_pci_add() for both stubdomain
(if applicable) and target domain.

> Yet I cannot see where the stubdomain
> libxl_domain_config will get the pci devices from the target domain
> assigned, I've looked in libxl__spawn_stub_dm but there doesn't seem
> to be any copy from the target to the stubdom of the list of pci
> devices. I guess I'm missing something.
> 
> Thanks, Roger.

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab
A: Because it messes up the order in which people normally read text.
Q: Why is top-posting such a bad thing?

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