Re: [RFC PATCH 17/17] KVM: PPC: Add an ioctl for userspace to select which platform to emulate

On Wed, Jun 29, 2011 at 01:56:16PM +0200, Alexander Graf wrote:

On 29.06.2011, at 13:53, Josh Boyer wrote:

quoted

On Wed, Jun 29, 2011 at 08:41:03PM +1000, Paul Mackerras wrote:

quoted

Documentation/virtual/kvm/api.txt   |   35 +++++++++++++++++++++++++++++++++++
arch/powerpc/include/asm/kvm.h      |   15 +++++++++++++++
arch/powerpc/include/asm/kvm_host.h |    1 +
arch/powerpc/kvm/powerpc.c          |   28 ++++++++++++++++++++++++++++
include/linux/kvm.h                 |    1 +
5 files changed, 80 insertions(+), 0 deletions(-)

diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
index b0e4b9c..3ab012c 100644
--- a/Documentation/virtual/kvm/api.txt
+++ b/Documentation/virtual/kvm/api.txt

@@ -1430,6 +1430,41 @@ is supported; 2 if the processor requires all virtual machines to have

an RMA, or 1 if the processor can use an RMA but doesn't require it,
because it supports the Virtual RMA (VRMA) facility.

+4.64 KVM_PPC_SET_PLATFORM
+
+Capability: none
+Architectures: powerpc
+Type: vm ioctl
+Parameters: struct kvm_ppc_set_platform (in)
+Returns: 0, or -1 on error
+
+This is used by userspace to tell KVM what sort of platform it should
+emulate.  The return value of the ioctl tells userspace whether the
+emulation it is requesting is supported by KVM.
+
+struct kvm_ppc_set_platform {
+	__u16 platform;		/* defines the OS/hypervisor ABI */
+	__u16 guest_arch;	/* e.g. decimal 206 for v2.06 */
+	__u32 flags;
+};
+
+/* Values for platform */
+#define KVM_PPC_PV_NONE		0	/* bare-metal, non-paravirtualized */
+#define KVM_PPC_PV_KVM		1	/* as defined in kvm_para.h */
+#define KVM_PPC_PV_SPAPR	2	/* IBM Server PAPR (a la PowerVM) */
+
+/* Values for flags */
+#define KVM_PPC_CROSS_ARCH	1	/* guest architecture != host */
+
+The KVM_PPC_CROSS_ARCH bit being 1 indicates that the guest is of a
+sufficiently different architecture to the host that the guest cannot
+be permitted to use supervisor mode.  For example, if the host is a
+64-bit machine and the guest is a 32-bit machine, then this bit should
+be set.

This makes me wonder if a similar thing might eventually be usable for
running an i686 or x32 guest on an x86_64 KVM host.  I have no idea if
that is even theoretically possible, but if it is it might be better to
rename the ioctl to be architecture agnostic.

On x86 this is not required unless we want to "virtualize" pre-CPUID CPUs. Everything as of Pentium has a full bitmap of feature capabilities that KVM gets from user space, including information such as "Can we do 64-bit mode?".

Ah.  Thank you for the explanation.

josh