[Xen-devel] Xen's Linux kernel config options V2

[Juergen Gross <jgross@xxxxxxxx>]

Hi,

This is a design proposal for a rework of the config options on the
Linux kernel which are related to Xen.

The need to do so arose from the fact that it is currently not
possible to build the Xen frontend drivers for a non-pvops kernel,
e.g. to run them in a HVM-domain. There are more drawbacks in the
current config options to which I'll come later.

Current Xen related config options are as follows:

Option                          Selects                 Depends
---------------------------------------------------------------------
XEN                             PARAVIRT_CLOCK(x86)     PARAVIRT(x86)
                                XEN_HAVE_PVMMU(x86)
                                SWIOTLB_XEN(arm,arm64)
  PCI_XEN(x86)                  SWIOTLB_XEN
  XEN_DOM0                                              PCI_XEN(x86)
    XEN_BACKEND
      XEN_BLKDEV_BACKEND
      XEN_PCIDEV_BACKEND(x86)
      XEN_SCSI_BACKEND
      XEN_NETDEV_BACKEND
    XEN_ACPI_HOTPLUG_MEMORY                             XEN_STUB
    XEN_ACPI_HOTPLUG_CPU                                XEN_STUB
    XEN_MCE_LOG(x86)
  XEN_PVHVM(x86)
    XEN_PVH(x86)
  XEN_MAX_DOMAIN_MEMORY(x86)
  XEN_SAVE_RESTORE(x86)
  XEN_DEBUG_FS(x86)
  XEN_FBDEV_FRONTEND            XEN_XENBUS_FRONTEND
                                INPUT_XEN_KBDDEV_FRONTEND
  XEN_BLKDEV_FRONTEND           XEN_XENBUS_FRONTEND
  HVC_XEN
    HVC_XEN_FRONTEND            XEN_XENBUS_FRONTEND
  TCG_XEN                       XEN_XENBUS_FRONTEND
  XEN_PCIDEV_FRONTEND           PCI_XEN
                                XEN_XENBUS_FRONTEND
  XEN_SCSI_FRONTEND             XEN_XENBUS_FRONTEND
  INPUT_XEN_KBDDEV_FRONTEND     XEN_XENBUS_FRONTEND
  XEN_WDT
  XEN_BALLOON
    XEN_SELFBALLOONING                                  XEN_TMEM
    XEN_BALLOON_MEMORY_HOTPLUG
    XEN_SCRUB_PAGES
  XEN_DEV_EVTCHN
  XENFS                         XEN_PRIVCMD
    XEN_COMPAT_XENFS
  XEN_SYS_HYPERVISOR
  XEN_XENBUS_FRONTEND
  XEN_GNTDEV
  XEN_GRANT_DEV_ALLOC
  SWIOTLB_XEN
  XEN_TMEM(x86)
  XEN_PRIVCMD
  XEN_STUB(x86_64)                                      BROKEN
  XEN_ACPI_PROCESSOR(x86)
  XEN_HAVE_PVMMU
  XEN_EFI(x64)
  XEN_NETDEV_FRONTEND           XEN_XENBUS_FRONTEND

Legend:
- The first column are the Xen config options. Indentation in this
  column reflects the dependency between those options (e.g.
  XEN_SCSI_BACKEND depends on XEN_BACKEND, which in turn depends on
  XEN_DOM0, which depends on XEN).
- The second column shows the options which are selected automatically
  if the corresponding option in the first column is configured.
- The last column shows additional dependencies which can't be shown via
  the indentation level of the first column.
- Some options or dependencies are architecture specific, they are
  listed in brackets (e.g. XEN_TMEM which is available for x86 only).
- Non-Xen options are mentioned only if they seem to be really relevant,
  like e.g. PARAVIRT or BROKEN.

There are several reasons to change the config options:
- Be able to build Xen frontend drivers for HVM domains without the need
  for choosing a pvops kernel: currently the frontend drivers need XEN
  configured which depends on PARAVIRT.
- Be able to build a Dom0 using XEN_PVH without having to configure
  XEN_HAVE_PVMMU.
- Be able to build HVM driver domains.
- Some features are available for x86 only, in spite of being not
  architecture specific, e.g. XEN_DEBUG_FS or XEN_TMEM.

After some feedback for the first draft I'd suggest the following:

Option                          Selects                 Depends
----------------------------------------------------------------------
XEN
  XEN_PV(x86)                   XEN_HAVE_PVMMU
                                PARAVIRT
                                PARAVIRT_CLOCK
  XEN_PVH(x86)                  XEN_PVHVM
                                PARAVIRT
                                PARAVIRT_CLOCK
  XEN_PVHVM                     PARAVIRT
                                PARAVIRT_CLOCK
  XEN_BACKEND                   SWIOTLB_XEN(arm,arm64)  XEN_PV(x86) ||
                                                        XEN_PVH(x86) ||
                                                        XEN_PVHVM
    XEN_BLKDEV_BACKEND
    XEN_PCIDEV_BACKEND(x86)
    XEN_SCSI_BACKEND
    XEN_NETDEV_BACKEND
  PCI_XEN(x86)                  SWIOTLB_XEN
  XEN_DOM0                      XEN_BACKEND             XEN_PV(x86) ||
                                PCI_XEN(x86)            XEN_PVH(x86)
    XEN_ACPI_HOTPLUG_MEMORY                             XEN_STUB
    XEN_ACPI_HOTPLUG_CPU                                XEN_STUB
    XEN_MCE_LOG(x86)
  XEN_MAX_DOMAIN_MEMORY(x86)
  XEN_SAVE_RESTORE(x86)
  XEN_DEBUG_FS
  XEN_WDT
  XEN_BALLOON
    XEN_SELFBALLOONING                                  XEN_TMEM
    XEN_BALLOON_MEMORY_HOTPLUG
    XEN_SCRUB_PAGES
  XENFS                         XEN_PRIVCMD
    XEN_COMPAT_XENFS
  XEN_SYS_HYPERVISOR
  XEN_DEV_EVTCHN
  XEN_GNTDEV
  XEN_GRANT_DEV_ALLOC
  SWIOTLB_XEN
  XEN_TMEM
  XEN_PRIVCMD
  XEN_STUB(x86_64)                                      BROKEN
  XEN_ACPI_PROCESSOR(x86)
  XEN_HAVE_PVMMU
  XEN_EFI(x64)
  XEN_XENBUS_FRONTEND
XEN_FRONTEND                    XEN
                                XEN_XENBUS_FRONTEND
  XEN_FBDEV_FRONTEND            INPUT_XEN_KBDDEV_FRONTEND
  XEN_BLKDEV_FRONTEND
  HVC_XEN_FRONTEND              HVC_XEN
  TCG_XEN
  XEN_PCIDEV_FRONTEND           PCI_XEN
  XEN_SCSI_FRONTEND
  INPUT_XEN_KBDDEV_FRONTEND
  XEN_NETDEV_FRONTEND
  XEN_PLATFORM_PCI

The main difference to the current status is the XEN setting no longer
controlling all other options.

Changing the config options in this way surely will need some
adjustments in the drivers, but this can be discussed when we agree
on the future config dependencies.


Changes in V2:
- XEN doesn't select SWIOTLB_XEN on arm, this is done by XEN_BACKEND now
  (David Vrabel and Stefano Stabellini)
- XEN_PVHVM now under XEN, selects PARAVIRT and PARAVIRT_CLOCK (David)
- XEN_FRONTEND independent from any XEN_* variant, selects XEN and
  XEN_XENBUS_FRONTEND (David, Juergen)
- added XEN_PLATFORM_PCI (David)

Juergen

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