[PATCH v9 0/7] KVM: x86: Allow Qemu/KVM to use PVH entry point

Maran Wilson
2018-12-10T19:07:59Z

For certain applications it is desirable to rapidly boot a KVM virtual
machine. In cases where legacy hardware and software support within the
guest is not needed, Qemu should be able to boot directly into the
uncompressed Linux kernel binary without the need to run firmware.

There already exists an ABI to allow this for Xen PVH guests and the ABI
is supported by Linux and FreeBSD:

https://xenbits.xen.org/docs/unstable/misc/pvh.html

This patch series would enable Qemu to use that same entry point for
booting KVM guests.

Changes from v8:

* Removed unused KVM_GUEST_PVH symbol.

Changes from v7:

(No functional changes from v7 other than rebasing to latest upstream)
* Added Review-by tags as provided by Juergen Gross (1,2,3,6,7)
* Rebasing to upstream 4.18 caused a minor conflict in patch 4 that had
to be hand merged due to this patch:
1fe8388 xen: share start flags between PV and PVH
I just had to make sure we were accounting for the xen_start_flags
in the new code path.
* Rebasing to upstream 4.20-rc4 caused a few minor conflicts in patches
2,3,5,7 that needed to be resolved by hand. The conflicts were due to
upstream non-functional code cleanup changes in arch/x86/xen/Makefile
and arch/x86/platform/pvh/enlighten.c due to these patches:
28c11b0 x86/xen: Move pv irq related functions under CONFIG_XEN_PV
umbrella
357d291 x86/xen: Fix boot loader version reported for PVH guests
3cfa210 xen: don't include <xen/xen.h> from <asm/io.h> and
<asm/dma-mapping.h>
* Qemu and qboot RFC patches have been posted to show one example of how
this functionality can be used. Some preliminary numbers are available
in those cover letters showing the KVM guest boot time improvement.
Qemu:
http://lists.nongnu.org/archive/html/qemu-devel/2018-12/msg00957.html
qboot:
http://lists.nongnu.org/archive/html/qemu-devel/2018-12/msg00953.html

Changes from v6:

* Addressed issues caught by the kbuild test robot:
- Restored an #include line that had been dropped by mistake (patch 4)
- Removed a pair of #include lines that were no longer needed in a
common code file and causing problems for certain 32-bit configs
(patchs 4 and 7)

Changes from v5:

* The interface changes to the x86/HVM start info layout have
now been accepted into the Xen tree.
* Rebase and merge upstream PVH file changes.
* (Patch 6) Synced up to the final version of the header file that was
acked and pulled into the Xen tree.
* (Patch 1) Fixed typo and removed redundant "def_bool n" line.

Changes from v4:

Note: I've withheld Juergen's earlier "Reviewed-by" tags from patches
1 and 7 since there were minor changes (mostly just addition of
CONFIG_KVM_GUEST_PVH as requested) that came afterwards.

* Changed subject prefix from RFC to PATCH
* Added CONFIG_KVM_GUEST_PVH as suggested
* Relocated the PVH common files to
arch/x86/platform/pvh/{enlighten.c,head.S}
* Realized I also needed to move the objtool override for those files
* Updated a few code comments per reviewer feedback
* Sent out a patch of the hvm_start_info struct changes against the Xen
tree since that is the canonical copy of the header. Discussions on
that thread have resulted in some (non-functional) updates to
start_info.h (patch 6/7) and those changes are reflected here as well
in order to keep the files in sync. The header file has since been
ack'ed for the Xen tree by Jan Beulich.

Changes from v3:

* Implemented Juergen's suggestion for refactoring and moving the PVH
code so that CONFIG_XEN is no longer required for booting KVM guests
via the PVH entry point.
Functionally, nothing has changed from V3 really, but the patches
look completely different now because of all the code movement and
refactoring. Some of these patches can be combined, but I've left
them very small in some cases to make the refactoring and code
movement easier to review.
My approach for refactoring has been to create a PVH entry layer that
still has understanding and knowledge about Xen vs non-Xen guest types
so that it can make run time decisions to handle either case, as
opposed to going all the way and re-writing it to be a completely
hypervisor agnostic and architecturally pure layer that is separate
from guest type details. The latter seemed a bit overkill in this
situation. And I've handled the complexity of having to support
Qemu/KVM boot of kernels compiled with or without CONFIG_XEN via a
pair of xen specific __weak routines that can be overridden in kernels
that support Xen guests. Importantly, the __weak routines are for
xen specific code only (not generic "guest type" specific code) so
there is no clashing between xen version of the strong routine and,
say, a KVM version of the same routine. But I'm sure there are many
ways to skin this cat, so I'm open to alternate suggestions if there
is a compelling reason for not using __weak in this situation.

Changes from v2:

* All structures (including memory map table entries) are padded and
aligned to an 8 byte boundary.

* Removed the "packed" attributes and made changes to comments as
suggested by Jan.

Changes from v1:

* Adopted Paolo's suggestion for defining a v2 PVH ABI that includes the
e820 map instead of using the second module entry to pass the table.

* Cleaned things up a bit to reduce the number of xen vs non-xen special
cases.

Maran Wilson (7):
xen/pvh: Split CONFIG_XEN_PVH into CONFIG_PVH and CONFIG_XEN_PVH
xen/pvh: Move PVH entry code out of Xen specific tree
xen/pvh: Create a new file for Xen specific PVH code
xen/pvh: Move Xen specific PVH VM initialization out of common file
xen/pvh: Move Xen code for getting mem map via hcall out of common
file
xen/pvh: Add memory map pointer to hvm_start_info struct
KVM: x86: Allow Qemu/KVM to use PVH entry point

MAINTAINERS | 1 +
arch/x86/Kbuild | 2 +
arch/x86/Kconfig | 6 ++
arch/x86/kernel/head_64.S | 2 +-
arch/x86/platform/pvh/Makefile | 5 +
arch/x86/platform/pvh/enlighten.c | 137 ++++++++++++++++++++++++
arch/x86/{xen/xen-pvh.S => platform/pvh/head.S} | 0
arch/x86/xen/Kconfig | 3 +-
arch/x86/xen/Makefile | 2 -
arch/x86/xen/enlighten_pvh.c | 94 ++++------------
include/xen/interface/hvm/start_info.h | 63 ++++++++++-
include/xen/xen.h | 3 +
12 files changed, 237 insertions(+), 81 deletions(-)
create mode 100644 arch/x86/platform/pvh/Makefile
create mode 100644 arch/x86/platform/pvh/enlighten.c
rename arch/x86/{xen/xen-pvh.S => platform/pvh/head.S} (100%)

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2.16.1