Anthony Liguori wrote: > Gregory Haskins wrote: >> >> So, yes, the delta from PIO to HC is 350ns. Yes, this is a ~1.4% >> improvement. So what? Its still an improvement. If that improvement >> were for free, would you object? And we all know that this change isn't >> "free" because we have to change some code (+128/-0, to be exact). But >> what is it specifically you are objecting to in the first place? Adding >> hypercall support as an pv_ops primitive isn't exactly hard or complex, >> or even very much code. >> > > Where does 25us come from? The number you post below are 33us and 66us. <snip> The 25us is approximately the max from an in-kernel harness strapped directly to the driver gathered informally during testing. The 33us is from formally averaging multiple runs of a userspace socket app in preparation for publishing. I consider the 25us the "target goal" since there is obviously overhead that a socket application deals with that theoretically a guest bypasses with the tap-device. Note that the socket application itself often sees < 30us itself...this was just a particularly "slow" set of runs that day. Note that this is why I express the impact as "approximately" (e.g. "~4%"). Sorry for the confusion. -Greg
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