On Thu, Oct 01, 2015 at 02:17:49PM -0700, Alexander Duyck wrote: > 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
VMs are running with IOMMU enabled anyway. Avi here tells us no one uses SRIOV on bare metal so ... we don't need to argue about that. -- MST