Re: [Xen-devel] additional domain.c memory allocation causes "xm create" to fail

[misiu godfrey <godfrey@xxxxxxxxxxxxx>]

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.

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.

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