Re: [Xen-devel] [PATCH 2 of 2] vpmu: Add the BTS extension

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 14.02.12 at 15:30, Dietmar Hahn <dietmar.hahn@xxxxxxxxxxxxxx> wrote:
> Am Dienstag 14 Februar 2012, 13:27:08 schrieb Jan Beulich:
>> Plus enforcing the buffer requirements to avoid CPU deadlock
>> (contiguous present pages, alignment). Failure to do so can hang the
>> CPU, and hence would represent a DoS vulnerability.
> 
> I'm not sure what you mean here. Are you speaking about the DS buffer?
> If yes, this is no problem, because the DS buffer addressm must be a domU
> virtual address. The processor only writes data into the buffer, if the
> domU is running so in the worst case the domU gets triggered a page fault
> or what I testet a triple fault occurs and the domU gets rebootet.

This certainly can be CPU model dependent, but on raw hardware
I know that not meeting the buffer constraints can hang (not triple
fault, perhaps a live lock) a CPU. Therefore, unless you can prove
this is impossible when running in VMX non-root, you will have to add
provisions for this (and this was the major reason keeping me from
trying to add DS support a year or two ago). At the very minimum
the whole functionality would otherwise need to be disabled by
default, and when enabled a prominent warning be issued (along
the lines of that of sync_console).

Jan


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