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Re: [vfio-users] [FEEDBACK NEEDED] Rewriting the Arch wiki article



"options kvm ignore_msrs=1"

On the wiki page as it stands now there is a warning, "Silently ignoring unknown MSR accesses could potentially break other software within the VM or other VMs." I'm not clear on all of the ramifications, though it came up on the mailing list before. I basically decided "not important enough for me to do it" and forgot all the details. But I wouldn't remove this warning or suggest this as a performance gain unless the risks have been eliminated (or are only applicable to specific edge cases or something).

On Tue, Apr 12, 2016 at 2:35 PM, Blank Field <ihatethisfield gmail com> wrote:

TL;DR point 2. is wrong, pcie-port, vfio-pci or pci-stub are allowed, see "IOMMU groups inside and out" blog post.

On Apr 12, 2016 11:30 PM, "Bronek Kozicki" <brok spamcop net> wrote:
On 12/04/2016 20:36, Nicolas Roy-Renaud wrote:
I've already rewritten the first two sections ("Prerequisites" and
"Setting up IOMMU"), and the rest of the article should essentially
follow the same basic structure and style. Replies here or on the wiki's
discussion page would be much appreciated.

Hope Alex can clear any misconception I am about to present below


1. I am using option iommu=pt, under impression this is expected to improve performance (my CPU is Xeon IvyBridge)

2. does PCI bridge have to be in a separate IOMMU group than passed-through device?

3. would be nice to provide hints for headless host. FWIW, I use combination of
3.1. kernel options:
console=ttyS0,115200N8R nomodest video=vesa:off video=efifb:off vga=normal
3.2.following line in /etc/modprobe.d/vfio.conf:
options vfio-pci disable_vga=1
3.3. large list of blacklisted modules (all framebuffers and nvidia and AMD drivers) in /etc/modprobe.d/blacklist.conf:
# This host is headless, prevent any modules from attaching to video hardware
# NVIDIA
blacklist nouveau
blacklist nvidia
# AMD
blacklist radeon
blacklist amdgpu
blacklist amdkfd
blacklist fglrx
# HDMI sound on a GPU
blacklist snd_hda_intel
# Framebuffers (ALL of them)
blacklist vesafb
blacklist aty128fb
blacklist atyfb
blacklist radeonfb
blacklist cirrusfb
blacklist cyber2000fb
blacklist cyblafb
blacklist gx1fb
blacklist hgafb
blacklist i810fb
blacklist intelfb
blacklist kyrofb
blacklist lxfb
blacklist matroxfb_base
blacklist neofb
blacklist nvidiafb
blacklist pm2fb
blacklist rivafb
blacklist s1d13xxxfb
blacklist savagefb
blacklist sisfb
blacklist sstfb
blacklist tdfxfb
blacklist tridentfb
blacklist vfb
blacklist viafb
blacklist vt8623fb
blacklist udlfb

4. ignore_msrs=1 also helps running Linux guests

5. do not use qemu:arg for binding host device to guest, here is example how to do it properly:
    <hostdev mode='subsystem' type='pci' managed='yes'>
      <driver name='vfio'/>
      <source>
        <address domain='0x0000' bus='0x82' slot='0x00' function='0x0'/>
      </source>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x08' function='0x0'/>
    </hostdev>
5.1. for nVidia Quadro, add just below </source ...>
<rom bar='off'/>

6. if guest is started from BIOS rather than UEFI, keep the above <hostdev ...> but replace emulator with a script, e.g.
# virsh dumpxml gdynia-vfio1 | grep emulator
    <emulator>/usr/bin/qemu-system-x86_64.xvga.sh</emulator>
# cat /usr/bin/qemu-system-x86_64.xvga.sh
#!/bin/sh
exec nice --adjustment=-5 /usr/bin/qemu-system-x86_64 `echo "$@" | \
    sed 's/-device vfio-pci,host=82:00.0/-device vfio-pci,host=82:00.0,x-vga=on/g' | \
    sed 's/-device vfio-pci,host=03:00.0/-device vfio-pci,host=03:00.0,x-vga=on/g'`


7. performance optimizations
7.1. use huge pages
7.2. use isolcpus
7.3. use vCPU pinnig
7.4. use virtio-scsi with multiple queues (depending on number of available CPUs, after removing these dedicated to guest)
7.5. use multiple queues for virtio-net
7.6. for Linux  guests, use P9 for mounting host filesystems in guest




B.

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