Re: [Xen-devel] [PATCH 05/11] xen pci platform device driver

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

On 05/10/2010 07:20 AM, Stefano Stabellini wrote:
> Add the xen pci platform device driver that is responsible
> for initializing the grant table and xenbus in PV on HVM mode.
> Few changes to xenbus and grant table are necessary to allow the delayed
> initialization in HVM mode.
> Grant table needs few additional modifications to work in HVM mode.
>
> When running on HVM the event channel upcall is never called while in
> progress because it is a normal Linux irq handler, therefore we cannot
> be sure that evtchn_upcall_pending is 0 when returning.
>   

Won't the event (and the interrupt it raises) still be pending when the
handler returns?

> For this reason if evtchn_upcall_pending is set by Xen we need to loop
> again on the event channels set pending otherwise we might loose some
> event channel deliveries.
>   

See below...

> Signed-off-by: Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>
> Signed-off-by: Sheng Yang <sheng@xxxxxxxxxxxxxxx>
> ---
>  drivers/xen/Kconfig                  |   11 ++-
>  drivers/xen/Makefile                 |    3 +-
>  drivers/xen/events.c                 |    5 +-
>  drivers/xen/grant-table.c            |   70 +++++++++-
>  drivers/xen/platform-pci.c           |  236 
> ++++++++++++++++++++++++++++++++++
>  drivers/xen/xenbus/xenbus_probe.c    |   20 ++-
>  include/xen/grant_table.h            |    1 +
>  include/xen/interface/grant_table.h  |    1 +
>  include/xen/interface/platform_pci.h |   45 +++++++
>  include/xen/platform_pci.h           |   41 ++++++
>  include/xen/xenbus.h                 |    1 +
>  11 files changed, 417 insertions(+), 17 deletions(-)
>  create mode 100644 drivers/xen/platform-pci.c
>  create mode 100644 include/xen/interface/platform_pci.h
>  create mode 100644 include/xen/platform_pci.h
>
> diff --git a/drivers/xen/Kconfig b/drivers/xen/Kconfig
> index cab100a..3e02457 100644
> --- a/drivers/xen/Kconfig
> +++ b/drivers/xen/Kconfig
> @@ -60,4 +60,13 @@ config XEN_SYS_HYPERVISOR
>           Create entries under /sys/hypervisor describing the Xen
>        hypervisor environment.  When running native or in another
>        virtual environment, /sys/hypervisor will still be present,
> -      but will have no xen contents.
> \ No newline at end of file
> +      but will have no xen contents.
> +
> +config XEN_PLATFORM_PCI
> +     tristate "xen platform pci device driver"
> +     depends on XEN
> +     help
> +       Driver for the Xen PCI Platform device: it is responsible for
> +       initializing xenbus and grant_table when running in a Xen HVM
> +       domain. As a consequence this driver is required to run any Xen PV
> +       frontend on Xen HVM.
> diff --git a/drivers/xen/Makefile b/drivers/xen/Makefile
> index 7c28434..2a502d2 100644
> --- a/drivers/xen/Makefile
> +++ b/drivers/xen/Makefile
> @@ -9,4 +9,5 @@ obj-$(CONFIG_XEN_XENCOMM)     += xencomm.o
>  obj-$(CONFIG_XEN_BALLOON)    += balloon.o
>  obj-$(CONFIG_XEN_DEV_EVTCHN) += evtchn.o
>  obj-$(CONFIG_XENFS)          += xenfs/
> -obj-$(CONFIG_XEN_SYS_HYPERVISOR)     += sys-hypervisor.o
> \ No newline at end of file
> +obj-$(CONFIG_XEN_SYS_HYPERVISOR)     += sys-hypervisor.o
> +obj-$(CONFIG_XEN_PLATFORM_PCI)               += platform-pci.o
> diff --git a/drivers/xen/events.c b/drivers/xen/events.c
> index cd609f4..197ccbc 100644
> --- a/drivers/xen/events.c
> +++ b/drivers/xen/events.c
> @@ -662,7 +662,7 @@ void __xen_evtchn_do_upcall(struct pt_regs *regs)
>  
>               count = __get_cpu_var(xed_nesting_count);
>               __get_cpu_var(xed_nesting_count) = 0;
> -     } while(count != 1);
> +     } while(count != 1 || vcpu_info->evtchn_upcall_pending);
>   

So the intention here is to pick up newly posted events while the event
processing loop is running?  But if this is necessary it won't work,
because it is still racy.  For example, what happens if a new event is
posted right here, just after the loop has exited?  Will it get
ignored?  Or if it does need to be processed, why is the test in the
loop necessary?

    J

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