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

"options kvm ignore_msrs=1"

If you guys don't need it that is because you don't game.
Launch metal gear solid phantom pain on a windows guest without this option and your virtual machine will crash, period.
This is mandatory for every gamer who plays recent games.

2016-04-12 23:24 GMT+02:00 Alex Williamson <alex l williamson gmail com>:
On Tue, Apr 12, 2016 at 2:30 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)

It only affects the performance of host devices.  There's less latency if host DMAs don't do dynamic mappings through the IOMMU, also less isolation between drivers.
2. does PCI bridge have to be in a separate IOMMU group than passed-through device?

No.  Blank is mostly correct on this, newer kernel remove the pcieport driver test and presumes any driver attached to a bridge device is ok.
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
blacklist nouveau
blacklist nvidia
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

I suspect that blacklisting framebuffer drivers doesn't actually do anything.
4. ignore_msrs=1 also helps running Linux guests

Never been needed in my experience.
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'/>
        <address domain='0x0000' bus='0x82' slot='0x00' function='0x0'/>
      <address type='pci' domain='0x0000' bus='0x00' slot='0x08' function='0x0'/>
5.1. for nVidia Quadro, add just below </source ...>
<rom bar='off'/>

Shouldn't be necessary to hide the ROM, Quadro should never be the primary graphics device and thus its ROM should never get executed.

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
# cat /usr/bin/qemu-system-x86_64.xvga.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

There are numerous ways to do this, that's one.   Hard to make any universal recommendation there, NFS, sshfs, SMB are also options.

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