Re: [PATCH for-4.5 v6 08/16] xen: Add limited support of VMware's hyper-call rpc

[Don Slutz <dslutz@verizon.com>]

On 09/22/14 09:47, Ian Campbell wrote:
> On Sat, 2014-09-20 at 14:07 -0400, Don Slutz wrote:
>> This interface is an extension of __HYPERVISOR_HVM_op.  It was
>> picked because xc_get_hvm_param() also uses it and VMware guest
>> info is a lot like a hvm param.
> Sorry if this has been discussed before, but did you consider doing all
> this in qemu rather than Xen?


Yes, but QEMU does not have access to the VCPU registers.  Also QEMU 
does not have this code; so would also need it to be added.

A second part would be adding a way for a tool stack to
communicate with QEMU to access this data.  xenstore/xenbus might
be one way to do this.

> Unless there are frequent accesses to these things then qemu would be
> the default best place for this sort of thing, especially since as
> you've observed there is some pretty complex memory management and
> string handling which it would generally be better to avoid in the
> hypervisor.

The more complex memory management is around not using the
heap during a single guest instruction (inl (%dx)).  Handling
migration of a fully variable sized hunk is not that simple.

Also preventing the guest from accessing outside of the defined
memory does increase the complexity.


> Your description of HVM_PARAM_VMPORT_RESET_TIME suggests they aren't
> typically accessed very frequently.

The rate I have seen is a low number per minute.  However there
is nothing that prevents a guest from doing a very high rate.

     -Don Slutz

> Ian.
>