Re: Adding new custom devices to Xen via QEMU

[Jason Dickens <jdickens@grammatech.com>]

Hi Wei,

Thanks for the response. It make sense to me that if the device were on 
the PCI bus (or other such bus, e.g. USB) that it could be discovered, 
at least by an OS. Its something to consider. I should mention that our 
guest VM doesn't actually use an OS.

However, the device is not implemented that as PCI it is simply memory 
mapped. Technically, in QEMU is has type ISA because it was derived as a 
modification of the TPM device. Is it possible something is lacking in 
the QEMU model that Xen needs but KVM doesn't?
If the answer is that Xen should not need modification for any new 
devices then this gives me hope.  You've also inspired some things to 
try, like whether or not smaller modifications to the TPM device work. 
One change that is significant to mention is that the physical address 
range use is anomalous, by which I mean it not in the normal device range.

Any references you could give how Xen actually discovers when to use 
QEMU to service MMIO reads/writes would be useful too. My current 
understanding is that "something" would have to be used to configure the 
EPT to trigger QEMU to use on VMEXITs caused by such accesses.

Thanks,

Jason

P.S. Regarding  device_model_args=
I'm already using this and QEMU seems quite satisfied as the device is 
realized as expected it simply doesn't receive verified MMIO accesses 
for the device.


On 9/30/2016 7:33 AM, Wei Liu wrote:
> Hello
>
> On Thu, Sep 29, 2016 at 09:35:21AM -0400, Jason Dickens wrote:
>> Hello,
>>
>> My name is Jason Dickens and I'm a Research Scientist here at GrammaTech.
>> Some of our research involves securing hypervisors and we have needed to add
>> to and/or modify Xen. I have been successful in modifying the source for
>> various purposes, but my question now is about devices. We have a custom
>> device model implemented in QEMU which works great with QEMU (on Intel)
>> standalone and with KVM, however, we now want access to it in Xen using the
>> same modified QEMU build.
>> The only problem I seem to be having is getting Xen to send the MMIO R/W's
>> to QEMU. The device is being realized, but guest access to the physical
>> address range I expect to reference the device seem to go no place. I see in
>> the source calls such as "register_io_handler" that other devices use to
>> effect the EPT mapping. Is this what I need?
>> My main question is whether or not it is truly necessary to change Xen
>> itself in order to introduce new devices in Xen using QEMU, or is there just
>> a configuration setting? And what is the simplest way to have a range of
> It is most likely achievable by just configuring your PCI device (?) in
> QEMU (provided it has a reasonable BAR size or whatnot). Check out
> various device model options in xl.cfg manpage, especially
> device_model_args= option allows you to add arbitrary options to QEMU.
>
> Wei.
>
>> physical addresses access a custom QEMU device?
>>
>> Thanks,
>> Jason
>>
>>
>> _______________________________________________
>> Xen-devel mailing list
>> Xen-devel@lists.xen.org
>> https://lists.xen.org/xen-devel



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