On 04/09/12 20:45, misiu godfrey wrote:
For what
purpose? There was once a bug which caused this to happen and
it caused Xen to slow to a glacial pace. We got bored of
debugging HVM
domains after several hours and the BIOS has still not loaded
the
bootloader.
I'm working on an experiment to do with cache-based side
channels in Cloud environments. Part of it involves measuring
the effect of flushing the cache every time there is a VM
switch.
Ok. My suspicion is that it will be unreasonably slow, although
should not be the glacial pace we saw with the bug (which was
flushing the L2 cache on every instruction).
You don't
check the return value, so what happens when the allocation
fails? I would say that calling *alloc() is not a sensible
thing to be
doing in __context_switch() anyway, as you are sitting doing a
long
operation while in a critical section of Xen code.
Unfortunately, I can chalk that up to my inexperience with
C programming. Thanks for pointing that out.
As for the sensibility of the plan, it is still in rather
early stages and not as robust as I would like it. As I get
more working I was planning on leaving the memory buffer
permanently allocated so as not to spend time managing it in a
critical section. If you have a suggestion for a more
practical solution I'm all ears.
Furthermore,
this algorithm has no guarantee to clear the L2 cache. In
fact is almost certainly will not.
This is the code that has worked in all of my prior
experiments and has been ratified by others I have worked
with. Are you sure it wouldn't work? While, for simplicity's
sake, I have removed portions of the code designed to
prevent pre-fetching and perhaps left out something important,
my understanding of cache-coloring, however, would still imply
that the data in the cache should be dirty, or flushed after
this loop terminates.
Perhaps I have misused the term "flush". My objective is
to make each cache line dirty, or flush it to main memory.
"flush" is the correct term.
However, the structure of caches work against you. With a set
associative cache, you have no control over which of the sets gets
used for your cache line. So on an N way set associate cache, your
worst case may only dirty 1/N of the actual lines in the cache.
After that, your L1 cache inclusion policy is going to affect how
you dirty your L2 cache, as well as whether you have joined caches
or split instruction and data caches.
Furthermore, on newer processes, multiple cores may share an L2 or
L3 cache, and context switches are unlike to occur at exactly the
same time on each core, meaning that a context switch on one core is
going to (attempt to) nuke the L2 cache of the VM which is in mid
run on another core. Conversely, even executing the loop trying to
dirty the cache will mean that you dont get all of it, and having
another core executing on the same L2 cache means it will pull its
data back during your dirtying loop.
--
Andrew Cooper - Dom0 Kernel Engineer, Citrix XenServer
T: +44 (0)1223 225 900, http://www.citrix.com
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