Re: [Xen-devel] [PATCH 1/3] libxl: attach xen-pciback only to PV domains

[Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>]

On Wed, Oct 19, 2016 at 04:46:26PM -0400, Konrad Rzeszutek Wilk wrote:
> On Wed, Oct 19, 2016 at 10:37:52AM +0100, Wei Liu wrote:
> > On Tue, Oct 18, 2016 at 03:53:31AM +0200, 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.
> > 
> > This bit of description is in line with my understanding of how PCI
> > passthrough works.
> 
> Kind of. Pciback is also used to "own" the PCI devices. And in fact
> they do an important job of resetting the PCI device when the
> device is "bind" to pciback:
> 
> echo <Bdf> > bind

This part is still done.

> And .. this is the important part - when device changes ownership.
> That is when you disconnect it from one guest and assign to another.
> You need to reset the device in between. The code that calls
> the pci_reset_function is called by:
> 
> }                                                                             
>   
>                                                                               
>   
> /*                                                                            
>   
>  * Called when:                                                               
>   
>  *  - XenBus state has been reconfigure (pci unplug). See 
> xen_pcibk_remove_device
>  *  - XenBus state has been disconnected (guest shutdown). See 
> xen_pcibk_xenbus_remove

But this, in case of HVM without stubdomain, is not.

>  *  - 'echo BDF > unbind' on pciback module with no guest attached. See 
> pcistub_remove
>  *  - 'echo BDF > unbind' with a guest still using it. See pcistub_remove     
>   
>  *                                                                            
>   
>  *  As such we have to be careful.                                            
>   
>  *                                                                            
>   
>  *  To make this easier, the caller has to hold the device lock.              
>   
>  */                                                                           
>   
> void pcistub_put_pci_dev(struct pci_dev *dev)
> 
> The first two are done when XenStore 'pci' entries are active - which
> this patch will remove and introduce a potential security problem.
> 
> Unless libxl does an 'unbind' followed by an 'bind'?

What about libxl__device_pci_reset, which is called (at least) before
attaching device to some domain, even after my patch and even if the
device is already bound to pciback. It tries to reset the device using
'reset' entry in sysfs. I see this isn't available for some devices -
can pci_reset_function do any better?


> 
> > 
> > > 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.
> > > 
> > > Signed-off-by: Marek Marczykowski-Górecki 
> > > <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
> > > ---
> > >  tools/libxl/libxl_pci.c | 5 +++--
> > >  1 file changed, 3 insertions(+), 2 deletions(-)
> > > 
> > > diff --git a/tools/libxl/libxl_pci.c b/tools/libxl/libxl_pci.c
> > > index 6f8f49c..2ae1bc4 100644
> > > --- a/tools/libxl/libxl_pci.c
> > > +++ b/tools/libxl/libxl_pci.c
> > > @@ -1111,7 +1111,7 @@ out:
> > >          }
> > >      }
> > >  
> > > -    if (!starting)
> > > +    if (!starting && !hvm)
> > >          rc = libxl__device_pci_add_xenstore(gc, domid, pcidev, starting);
> > >      else
> > >          rc = 0;
> > > @@ -1306,7 +1306,8 @@ static void libxl__add_pcidevs(libxl__egc *egc, 
> > > libxl__ao *ao, uint32_t domid,
> > >          }
> > >      }
> > >  
> > > -    if (d_config->num_pcidevs > 0) {
> > > +    if (d_config->num_pcidevs > 0
> > > +            && d_config->c_info.type == LIBXL_DOMAIN_TYPE_PV) {
> > 
> > Please move the indentation forward.
> > 
> > >          rc = libxl__create_pci_backend(gc, domid, d_config->pcidevs,
> > >              d_config->num_pcidevs);
> > >          if (rc < 0) {
> > > -- 
> > > 2.5.5
> > > 
> > 
> > _______________________________________________
> > Xen-devel mailing list
> > Xen-devel@xxxxxxxxxxxxx
> > https://lists.xen.org/xen-devel

-- 
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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