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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

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.

> 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@xxxxxxxxxxxxxxxxxxxxxx>
> ---
> 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. 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.

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