On 10/01/2015 02:42 AM, Michael S. Tsirkin wrote: > On Thu, Oct 01, 2015 at 12:22:46PM +0300, Avi Kivity wrote: >> even when they are some users >> prefer to avoid the performance penalty. > I don't think there's a measureable penalty from passing through the > IOMMU, as long as mappings are mostly static (i.e. iommu=pt). I sure > never saw any numbers that show such.
It depends on the IOMMU. I believe Intel had a performance penalty on all CPUs prior to Ivy Bridge. Since then things have improved to where they are comparable to bare metal. The graph on page 5 of https://networkbuilders.intel.com/docs/Network_Builders_RA_vBRAS_Final.pdf shows the penalty clear as day. Pretty much anything before Ivy Bridge w/ small packets is slowed to a crawl with an IOMMU enabled. - Alex
