[Xen-devel] Re: [PATCH] xen: core dom0 support

[Jeremy Fitzhardinge <jeremy@xxxxxxxx>]

Nick Piggin wrote:
> On Saturday 28 February 2009 17:52:24 Jeremy Fitzhardinge wrote:
>   
> > Andrew Morton wrote:
> >     
>   
> > > I hate to be the one to say it, but we should sit down and work out
> > > whether it is justifiable to merge any of this into Linux.  I think
> > > it's still the case that the Xen technology is the "old" way and that
> > > the world is moving off in the "new" direction, KVM?
> > >       
> > I don't think that's a particularly useful way to look at it.  They're
> > different approaches to the problem, and have different tradeoffs.
> >
> > The more important question is: are there real users for this stuff?
> > Does not merging it cause more net disadvantage than merging it?
> > Despite all the noise made about kvm in kernel circles, Xen has a large
> > and growing installed base.  At the moment its all running on massive
> > out-of-tree patches, which doesn't make anyone happy.  It's best that it
> > be in the mainline kernel.  You know, like we argue for everything else.
> >     
> OTOH, there are good reasons not to duplicate functionality, and many
> many times throughout the kernel history competing solutions have been
> rejected even though the same arguments could be made about them.
>
> There have also been many times duplicate functionality has been merged,
> although that does often start with the intention of eliminating
> duplicate implementations and ends with pain. So I think Andrew's
> question is pretty important.
>   
Those would be pertinent questions if I were suddenly popping up and 
saying "hey, let's add Xen support to the kernel!"  But Xen support has 
been in the kernel for well over a year now, and is widely used, enabled 
in distros, etc.  The patches I'm proposing here are not a whole new 
thing, they're part of the last 10% to fill out the kernel's support to 
make it actually useful.
> The user issue aside -- that is a valid point -- you don't really touch
> on the technical issues. What tradeoffs, and where Xen does better
> than KVM would be interesting to know, can Xen tools and users ever be
> migrated to KVM or vice versa (I know very little about this myself, so
> I'm just an interested observer).
>   
OK, fair point, its probably time for another Xen architecture refresher 
post.
There are two big architectural differences between Xen and KVM:

Firstly, Xen has a separate hypervisor who's primary role is to context 
switch between the guest domains (virtual machines).   The hypervisor is 
relatively small and single purpose.  It doesn't, for example, contain 
any device drivers or even much knowledge of things like pci buses and 
their structure.  The domains themselves are more or less peers; some 
are more privileged than others, but from Xen's perspective they are 
more or less equivalent.  The first domain, dom0, is special because its 
started by Xen itself, and has some inherent initial privileges; its 
main job is to start other domains, and it also typically provides 
virtualized/multiplexed device services to other domains via a 
frontend/backend split driver structure.
KVM, on the other hand, builds all the hypervisor stuff into the kernel 
itself, so you end up with a kernel which does all the normal kernel 
stuff, and can run virtual machines by making them look like slightly 
strange processes.
Because Xen is dedicated to just running virtual machines, its internal 
architecture can be more heavily oriented towards that task, which 
affects things from how its scheduler works, its use and multiplexing of 
physical memory.  For example, Xen manages to use new hardware 
virtualization features pretty quickly, partly because it doesn't need 
to trade-off against normal kernel functions.  The clear distinction 
between the privileged hypervisor and the rest of the domains makes the 
security people happy as well.  Also, because Xen is small and fairly 
self-contained, there's quite a few hardware vendors shipping it burned 
into the firmware so that it really is the first thing to boot (many of 
instant-on features that laptops have are based on Xen).  Both HP and 
Dell, at least, are selling servers with Xen pre-installed in the firmware.
The second big difference is the use of paravirtualization.  Xen can 
securely virtualize a machine without needing any particular hardware 
support.  Xen works well on any post-P6 or any ia64 machine, without 
needing any virtualzation hardware support.  When Xen runs a kernel in 
paravirtualized mode, it runs the kernel in an unprivileged processor 
state.  The allows the hypervisor to vet all the guest kernel's 
privileged operations, which are carried out are either via hypercalls 
or by memory shared between each guest and Xen.
By contrast, KVM relies on at least VT/SVM (and whatever the ia64 equiv 
is called) being available in the CPUs, and needs the most modern of 
hardware to get the best performance.
Once important area of paravirtualization is that Xen guests directly 
use the processor's pagetables; there is no shadow pagetable or use of 
hardware pagetable nesting.  This means that a tlb miss is just a tlb 
miss, and happens at full processor performance.  This is possible 
because 1) pagetables are always read-only to the guest, and 2) the 
guest is responsible for looking up in a table to map guest-local pfns 
into machine-wide mfns before installing them in a pte.  Xen will check 
that any new mapping or pagetable satisfies all the rules, by checking 
that the writable reference count is 0, and that the domain owns (or has 
been allowed access to) any mfn it tries to install in a pagetable.
The other interesting part of paravirtualization is the abstraction of 
interrupts into event channels.  Each domain has a bit-array of 1024 
bits which correspond to 1024 possible event channels.  An event channel 
can have one of several sources, such as a timer virtual interrupt, an 
inter-domain event, an inter-vcpu IPI, or mapped from a hardware 
interrupt.  We end up mapping the event channels back to irqs and they 
are delivered as normal interrupts as far as the rest of the kernel is 
concerned.
The net result is that a paravirtualized Xen guest runs a very close to 
full speed.  Workloads which modify live pagetables a lot take a bit of 
a performance hit (since the pte updates have to trap to the hypervisor 
for validation), but in general this is not a huge deal.  Hardware 
support for nested pagetables is only just beginning to get close to 
getting performance parity, but with different tradeoffs (pagetable 
updates are cheap, but tlb misses are much more expensive, and hits 
consume more tlb entries).
Xen can also make full use of whatever hardware virtualization features 
are available when running an "hvm" domain.  This is typically how you'd 
run Windows or other unmodified operating systems.
All of this is stuff that's necessary to support any PV Xen domain, and 
has been in the kernel for a long time now.
The additions I'm proposing now are those needed for a Xen domain to 
control the physical hardware, in order to provide virtual device 
support for other less-privileged domains.  These changes affect a few 
areas:
   * interrupts: mapping a device interrupt into an event channel for
     delivery to the domain with the device driver for that interrupt
   * mappings: allowing direct hardware mapping of device memory into a
     domain
   * dma: making sure that hardware gets programmed with machine memory
     address, nor virtual ones, and that pages are machine-contiguous
     when expected

Interrupts require a few hooks into the x86 APIC code, but the end 
result is that hardware interrupts are delivered via event channels, but 
then they're mapped back to irqs and delivered normally (they even end 
up with the same irq number as they'd usually have).
Device mappings are fairly easy to arrange.  I'm using a software pte 
bit, _PAGE_IOMAP, to indicate that a mapping is a device mapping.  This 
bit is set by things like ioremap() and remap_pfn_range, and the Xen mmu 
code just uses the pfn in the pte as-is, rather than doing the normal 
pfn->mfn translation.
DMA is handled via the normal DMA API, with some hooks to swiotlb to 
make sure that the memory underlying its pools is really DMA-ready (ie, 
is contiguous and low enough in machine memory).
The changes I'm proposing may look a bit strange from a purely x86 
perspective, but they fit in relatively well because they're not all 
that different from what other architectures require, and so the 
kernel-wide infrastructure is mostly already in place.
I hope that helps clarify what I'm trying to do here, and why Xen and 
KVM do have distinct roles to play.
   J

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