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[Xen-devel] Granularity of Credit and RTDS Scheduler


  • To: xen-devel@xxxxxxxxxxxxx
  • From: wy11 <wy11@xxxxxxxx>
  • Date: Fri, 06 Jan 2017 12:34:48 -0600
  • Delivery-date: Sun, 08 Jan 2017 07:03:51 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xen.org>

Dear Xen developers,

Recently I read a paper about possible theft of service attacks in Xen hypervisor.

https://arxiv.org/pdf/1103.0759.pdf

Due to the 10 ms intervals between sampling points, a malicious VM is able to run less than a interval and sleep to avoid being accounted.

According to the info page of RTDS, it seems that after V4.7, a RTDS based scheduler achieves a granularity of microsecond. However, is it able that a VM runs for less than a microsecond and relinquish the host actively so as to keep its budget?

A similar problem occurs in earlier Linux kernel, and it is fixed in today's Linux on x86 machines by utilizing a clock source TSC with a granularity of nanoseconds. I'd like to know if there is any reason that the Xen hypervisor does not choose a nanosecond scheduler?

Thanks a lot.

Best,

Wenqiu


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