Re: [Qemu-devel] [PATCH v2 5/6] backdoor: [i386] provide and implement intruction-based backdoor interface

[Lluís]

Gleb Natapov writes:

> On Thu, Nov 04, 2010 at 11:36:15PM +0100, Lluís wrote:
>> Take the unused CPUID 0x40001xxx range as the backdoor instruction.
>> 
> In KVM (and it fits the spec nicely) cpuid is defined in terms of
> tables.  There is no callback that is called when particular cpuid is
> queried, so such backdoor interface will be impossible to implement
> in KVM. Furthermore any interface that changes/looks at vcpu state in
> userspace is broken for KVM. Look at vmware backdoor interface for
> instance. KVM  has a hack in emulator code to make it work.

I know. I looked into the KVM implementation and neither CPUID nor
VMCALL/VMMCALL (these two are, in fact, obsoleted) are implemented as
calls to the hypervisor (although the hardware supports it).

The only interfaces exported by KVM to that purpose are through
MMIO/PIO, but these are OS-dependant (aka, KVM-dependant).

As such, there is currently no generic, OS-independant and low-overhead
method for providing a backdoor communication channel from the guest
directly into QEMU.

As I see this is very tied to my setup, I'll move this patch series down
below the tracing series so that it does not interfere with the other
patches.

Lluis

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