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RE: [Xen-devel] swiotlb and Xen query?

To: "pradeep singh rautela" <rautelap@xxxxxxxxx>
Subject: RE: [Xen-devel] swiotlb and Xen query?
From: "Petersson, Mats" <Mats.Petersson@xxxxxxx>
Date: Wed, 2 May 2007 15:42:31 +0200
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Thread-topic: [Xen-devel] swiotlb and Xen query?
Comments inline, but first let me just state something that may not be

The below comments apply to REAL hardware, meaning when a domain (either
Dom0 or another domain) is accessing for example a network controller or
disk-controller. This is not how most DomU's operate in the typical
setup - they use virtual devices. Virtual devices operate through
grant-tables and event-channels, and do not use swiotlb. 

Let me also clarify another thing: Dom0 is in no way special in any
other way than "it's privileged", so it's allows to do some of the
hypercalls that other domains can't use. But there's no difference from
the hypervisors perspective in how most of the hypercalls operate for
Dom0 and any other domain. For example, any domain can, technically,
access a hardware device. It just so happens that the traditional way to
configure Xen is to assign all the hardware to Dom0 and then use virtual
devices in other domains. But it's perfectly fine to hide any hardware
from Dom0 to assign it to any other domain - only hardware that Dom0
needs to have access to is the parts necessary for it to boot and
operate in itself, of course. Domains that have "hardware" are
considered "driver domains". 

> -----Original Message-----
> From: pradeep singh rautela [mailto:rautelap@xxxxxxxxx] 
> Sent: 02 May 2007 14:18
> To: Petersson, Mats
> Cc: xen-devel; xenkernelnewbies@xxxxxxxxxxxxxxxx
> Subject: Re: [Xen-devel] swiotlb and Xen query?
> Hi Mats,
> sorry again for complete newbie questions.
> Please find my further queries inline.
> On 5/2/07, Petersson, Mats < Mats.Petersson@xxxxxxx 
> <mailto:Mats.Petersson@xxxxxxx> > wrote:
>       > dom0 and domU uses swiotlb mechanism to emulate hardware 
>       > IOMMU, right?
>       > And that so because the whole memory is visible to hypervisor
>       > only and not to other domains, am i right?
>       Not so. It uses swiotlb to allow 32-bit devices to 
> access memory above 
>       4GB.
> Ok, perhaps i messed up the question.sorry for that.
> What i mean is, since a device has no idea of the mappings 
> between domU memory and the machine memory, does this means 
> the swiotlb is used to make sure that memory to which a 
> device has DMAed can be actually mapped to the correct domain? 

No. The device also has no idea about the virtual (as used by Windows or
Linux for example) to physical mapping in a "real" machine, right? So it
is the responsibility of the OS+driver to translate any virtual address
into a physical address. Since DomU (and Dom0) can translate it's own
memory addresses, this poses no problem - just take the virtual address
of the buffer(s) and translate it into physical address(es). The
physical address(es) are then given to the device itself to transfer. 

> Because device can DMA on to any 32 bit memory as you have 
> correctly stated, Mats and that is till 4GB of machine 
> memory. This memory to which the 32bit device can DMA need to 
> be given to the correct domU isn't it?Who makes sure this? 

At the moment, there's nothing preventing a DomU from DMA'ing to/from
the low 4GB of physical memory (and if the device allows it, all of the
physical memory in the machine). This is why Dom0 and any driver domain
is a potential security hole in Xen. This is of course no different from
how it works in any other system without Xen - there's no prevention of
a Windows disk driver writing the entire memory content to disk, for
example [in fact, this is almost exactly how Windows Hibernate works]. 

> swiotlb or hypervisor only using grant tables? If not how is 
> zero copying ensured, if it is at all in this case? 
>       >
>       > My doubt is once the translation table between virtual and
>       > physical addresses is set by the swiotlb.How does device
>       > becomes aware of this table which is emulated in hardware as
>       > a matter of fact using swiotlb.the device does not knows
>       > about this table when accessing physical memory, isnt it? 
>       It works because swiotlb isn't at all a translation. 
> Instead, it copies
>       the buffer of the requesting driver to a specifically 
> allocated memory
>       region below 4GB. 
> So does this means it copies the buffer from dom0 on to domU 
> using swiotlb whenever there is a DMA transfer? Isn't that an 
> overhead? copying ? 

There is no copy from Dom0 to DomU or other way around for driver
domains. The swiotlb uses a special modifier on the memory allocation
that it uses for "bounce buffers" to allocate memory that is in the low
region (below 4GB). 

But if the device does need to use memory in the low 4GB, and the buffer
isn't in the low 4GB, then the buffer content is copied to a temporary
buffer by swiotlb-code. And yes, there is some overhead in this. But
it's less overhead than not being able to transfer the data at all, and
relying on some person to walk with a piece of paper with the data
written down, and then writing the data into the other machine... ;-)

>       This new (low) memory location is what's being given
>       to the hardware device, so it never knew anything about 
> the buffer 
>       originally having been elsewhere.
>       >
>       > Another question is -
>       > dom0 and domU they both have their swiotlb mappings.This
>       > means Xen also keeps record of a mapping between these two
>       > tables in dom0 and domU? Am i right? Doesnt this means 
>       > wasting a lot of memory in allocating bounce buffers for
>       > swiotlb during booting of dom0 and then during 
> booting of domUs?
>       There are no mappings of memory within swiotlb.
>       And by the way, both Dom0 and DomU (in the para-virtual 
> case) knows 
>       exactly where the actual physical address is in the 
> machine - the
>       hypervisor checks when the guest writes to its 
> page-tables that the
>       memory is within that guest.
> Thanks a lot Mats but i  mean when the device writes to the memory. 

The device (in general) writes to memory that it's been told to write
to, by the OS in the Domain. And as described above, it's given the
physical address. 

Note that there are security issues with certain hardware that allows
"remote target addressing", meaning that the remote end can decide where
in the target system the data ends up. So, with the right hardware +
cabling and software, you can over-write (for example) the user-id in
the "who's logged in", so that you become "root" on a machine where you
don't have root privileges. Fortunately, this can not be done on
Ethernet controllers!

> Thank you once again.
> ~psr
>       --
>       Mats
>       >
>       > Please bear as i am just starting with the Xen code . 
>       > Any pointers or links will be really helpful.
>       >
>       > Thank you
>       > ~psr
>       >
>       >
>       > --
>       > ---
>       > pradeep singh rautela
>       >
>       > "Genius is 1% inspiration, and 99% perspiration" - not me :) 
>       >
> -- 
> ---
> pradeep singh rautela
> "Genius is 1% inspiration, and 99% perspiration" - not me :) 

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