[Xen-devel] Granularity of Credit and RTDS Scheduler

[wy11 <wy11@xxxxxxxx>]

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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