Re: [PATCH 01/16] KVM-HDR: register KVM basic header infrastructure

From: Avi Kivity
Date: Wed Jan 26 2011 - 06:06:54 EST

On 01/24/2011 08:06 PM, Glauber Costa wrote:
KVM, which stands for KVM Virtual Memory (I wanted to call it KVM Virtual Mojito),
is a piece of shared memory that is visible to both the hypervisor and the guest
kernel - but not the guest userspace.

The basic idea is that the guest can tell the hypervisor about a specific
piece of memory, and what it expects to find in there. This is a generic
abstraction, that goes to userspace (qemu) if KVM (the hypervisor) can't
handle a specific request, thus giving us flexibility in some features
in the future.

KVM (The hypervisor) can change the contents of this piece of memory at
will. This works well with paravirtual information, and hopefully
normal guest memory - like last update time for the watchdog, for

This is basic KVM registration headers. I am keeping headers
separate to facilitate backports to people who wants to backport
the kernel part but not the hypervisor, or the other way around.

Signed-off-by: Glauber Costa<glommer@xxxxxxxxxx>
CC: Avi Kivity<avi@xxxxxxxxxx>
arch/x86/include/asm/kvm_para.h | 11 +++++++++++
1 files changed, 11 insertions(+), 0 deletions(-)

diff --git a/arch/x86/include/asm/kvm_para.h b/arch/x86/include/asm/kvm_para.h
index a427bf7..b0b0ee0 100644
--- a/arch/x86/include/asm/kvm_para.h
+++ b/arch/x86/include/asm/kvm_para.h
@@ -21,6 +21,7 @@

/* The last 8 bits are used to indicate how to interpret the flags field
* in pvclock structure. If no bits are set, all flags are ignored.
@@ -35,6 +36,16 @@
#define MSR_KVM_SYSTEM_TIME_NEW 0x4b564d01
#define MSR_KVM_ASYNC_PF_EN 0x4b564d02

+#define MSR_KVM_REGISTER_MEM_AREA 0x4b564d03
+struct kvm_memory_area {
+ __u64 base;
+ __u32 size;
+ __u32 type;
+ __u8 result;
+ __u8 pad[3];

I'm guessing the protocol here is:

- guest fills in ->base/size/type
- issues wrmsr
- host registers the memory and updates ->result
- guest examines ->result

there are two issues with this approach:

- it doesn't lend itself will to live migration. Extra state must be maintained in the hypervisor.
- it isn't how normal hardware operates

what's wrong with extending the normal approach of one msr per feature?

error compiling committee.c: too many arguments to function

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