From: Arnd Bergmann <arnd@arndb.de>
To: Avi Kivity <avi@qumranet.com>
Subject: Re: [PATCH 0/7] KVM: Kernel-based Virtual Machine
Date: Sun, 22 Oct 2006 17:23:48 +0200
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Cc: Muli Ben-Yehuda <muli@il.ibm.com>,
       linux-kernel <linux-kernel@vger.kernel.org>,
       Anthony Liguori <aliguori@us.ibm.com>,
       Alan Cox <alan@lxorguk.ukuu.org.uk>
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On Sunday 22 October 2006 10:37, Avi Kivity wrote:
> I like this. ?Since we plan to support multiple vcpus per vm, the fs
> structure might look like:
>
> /kvm/my_vm
> ? ? |
> ? ? +----memory ? ? ? ? ?# mkdir to create memory slot.

Note that the way spufs does it, every directory is a reference-counted
object. Currently that includes single contexts and groups of
contexts that are supposed to be scheduled simultaneously.

The trick is that we use the special 'spu_create' syscall to
add a new object, while naming it, and return an open file
descriptor to it. When that file descriptor gets closed, the
object gets garbage-collected automatically.

This way you can simply kill a task, which also cleans up
all the special objects it allocated.

We ended up adding a lot more file than we initially planned,
but the interface is really handy, especially if you want to
create some procps-like tools for it.

> ? ? | ? ? | ? ? ? ? ? ? ?# ? ?how to set size and offset?
> ? ? | ? ? |
> ? ? | ? ? +---0 ? ? ? ? ?# guest physical memory slot
> ? ? | ? ? ? ? |
> ? ? | ? ? ? ? +-- dirty_bitmap ?# read to get and atomically reset
> ? ? | ? ? ? ? ? ? ? ? ? ? ? ? ? # the changed pages log

Have you thought about simply defining your guest to be a section
of the processes virtual address space? That way you could use
an anonymous mapping in the host as your guest address space, or
even use a file backed mapping in order to make the state persistant
over multiple runs. Or you could map the guest kernel into the
guest real address space with a private mapping and share the
text segment over multiple guests to save L2 and RAM.

> ? ? |
> ? ? |
> ? ? +----cpu ? ? ? ? ? ? # mkdir/rmdir to create/remove vcpu
> ? ? ? ? ? |

I'd recommend not allowing mkdir or similar operations, although
it's not that far off. One option would be to let the user specify
the number of CPUs at kvm_create() time, another option might
be to allow kvm_create with a special flag or yet another syscall
to create the vcpu objects.

> ? ? ? ? ? +----0
> ? ? ? ? ? | ? ? |
> ? ? ? ? ? | ? ? +--- irq ? ? # write to inject an irq
> ? ? ? ? ? | ? ? |
> ? ? ? ? ? | ? ? +--- regs ? ?# read/write to get/set registers
> ? ? ? ? ? | ? ? |
> ? ? ? ? ? | ? ? +--- debugger ? # write to set breakpoints/singlestep mode
> ? ? ? ? ? |
> ? ? ? ? ? +----1
> ? ? ? ? ? ? ? ? [...]
>
> It's certainly a lot more code though, and requires new syscalls. ?Since
> this is a little esoteric does it warrant new syscalls?

We've gone through a number of iterations on the spufs design regarding this,
and in the end decided that the garbage-collecting property of spu_create
was superior to any other option, and adding the spu_run syscall was then
the logical step. BTW, one inspiration for spu_run came from sys_vm86, which
as you are probably aware of is already doing a lot of what you do, just
not for protected mode guests.

	Arnd <><
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