Re: [Xen-devel] [PATCH for-4.5 v6 00/16] Xen VMware tools support

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 22.09.14 at 17:38, <george.dunlap@xxxxxxxxxxxxx> wrote:
> On 09/22/2014 04:34 PM, Ian Campbell wrote:
>> On Mon, 2014-09-22 at 16:19 +0100, George Dunlap wrote:
>>> On 09/22/2014 02:56 PM, Ian Campbell wrote:
>>>> On Sat, 2014-09-20 at 14:07 -0400, Don Slutz wrote:
>>>>
>>>>> I picked this subset to start with because it only has changes in
>>>>> Xen.
>>>>>
>>>>> Some of this code is already in QEMU
>>>> As I suggest in my reply to one for the rpc port patches it's not clear
>>>> that any of this needs to be in Xen rather than qemu in the first place.
>>>>
>>>> I came to think this even more once I saw the save/restore support...
>>> I don't think qemu can get notified on either cpuid or #GP faults, can it?
>> I understand the need for the cpuid bits, I should have made that clear.
>>
>>> A big chunk of the functionality here is to allow a userspace process to
>>> transparently make the "hypercalls" without the OS needing to explicitly
>>> give it access to the IO space, by trapping the resulting #GP faults and
>>> checking to see if they are IO instructions .  If that's functionality
>>> we think is important, then it will have to be done in Xen, I think.
>> Ah, the need to #GP was what I had missed, I was thinking it was just a
>> regular I/O port access.
>>
>> Having trapped the #GP and decoded it into an IO access, is there
>> anything stopping us forwarding that to qemu for consideration?
>>
>> (I confess I'm not sure why this is a #GP thing and not a VTd/SVM I/O
>> access trap, just like if userspace mmaps /dev/ioports, but I'll trust
>> that's just my lack of x86 hw virt knowledge)
> 
> I'm not 100% sure of this, but my understanding was that it *would* be a 
> normal IO trap *if* the guest OS gave access to that IO range to the 
> guest (via IOPL, maybe?).  But if the userspace program is not 
> explicitly given access by the OS to those ports, it will generate a #GP 
> instead.  The idea is to allow the "hypercall" to happen *without 
> cooperation* from the guest OS.
> 
> Again, that's my understanding, someone please correct me if I'm wrong...

That's indeed what was said so far. I wonder though whether opening
this up without guest OS consent isn't gong to introduce a security
issue inside the guest (depending on the exact functionality of these
hypercalls).

Jan


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