KVM with PCI forwarding really slow after 4.1

["Michael Büsch" <m@bues.ch>]

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Hi,

I use "-device pci-assign,host=00:1a.0" to forward a USB host chip to a
Win7 32 bit inside of qemu/kvm. That used to work pretty well, but it broke
horribly somewhere after 4.1. With recent kernels the virtual machine
boots, but is _very_ slow. It takes hours to boot.
If PCI forwarding is disabled, everything is fine.

qemu throws this warning on startup:
qemu-system-i386: -device pci-assign,host=00:1a.0: PCI region 0 at address 0xf253a000 has size 0x400, which is not a multiple of 4K.  You might experience some performance hit due to that.

_But_ it also shows that warning for 4.1 and earlier kernels that work pretty fast.

I tried to bisect the problem, but I ran into some some kernels that
don't even boot on my machine (the skipped ones). So it's a bit hard to
make progress.

Here is my git bisect log that narrows it down to under 100 commits.
Does anyone have a clue what could cause this?

(The log can be replayed with git bisect replay on Linus' tree).




# bad: [8005c49d9aea74d382f474ce11afbbc7d7130bec] Linux 4.4-rc1
# good: [b953c0d234bc72e8489d3bf51a276c5c4ec85345] Linux 4.1
git bisect start 'v4.4-rc1' 'v4.1'
# bad: [dd5cdb48edfd34401799056a9acf61078d773f90] Merge git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next
git bisect bad dd5cdb48edfd34401799056a9acf61078d773f90
# bad: [23908db413eccd77084b09c9b0a4451dfb0524c0] Merge tag 'staging-4.2-rc1' of git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging
git bisect bad 23908db413eccd77084b09c9b0a4451dfb0524c0
# bad: [14738e03312ff1137109d68bcbf103c738af0f4a] Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/dtor/input
git bisect bad 14738e03312ff1137109d68bcbf103c738af0f4a
# good: [5a602e157a9d91d5ce98d07c404097edba8ec9f3] Merge tag 'spi-v4.2' of git://git.kernel.org/pub/scm/linux/kernel/git/broonie/spi
git bisect good 5a602e157a9d91d5ce98d07c404097edba8ec9f3
# good: [a4244b0cf58d56c171874e85228ba5deffeb017a] net/ethtool: Add current supported tunable options
git bisect good a4244b0cf58d56c171874e85228ba5deffeb017a
# bad: [98ec21a01896751b673b6c731ca8881daa8b2c6d] Merge branch 'sched-hrtimers-for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
git bisect bad 98ec21a01896751b673b6c731ca8881daa8b2c6d
# good: [4b1f2af6752a4cc9acc1c22ddf3842478965f113] Merge branch 'for-linus' of git://git.kernel.org/pub/scm/linux/kernel/git/s390/linux
git bisect good 4b1f2af6752a4cc9acc1c22ddf3842478965f113
# good: [08d183e3c1f650b4db1d07d764502116861542fa] Merge tag 'powerpc-4.2-1' of git://git.kernel.org/pub/scm/linux/kernel/git/mpe/linux
git bisect good 08d183e3c1f650b4db1d07d764502116861542fa
# skip: [05fe125fa3237de2ec5bada80031e694de78909c] Merge tag 'kvm-arm-for-4.2' of git://git.kernel.org/pub/scm/linux/kernel/git/kvmarm/kvmarm into HEAD
git bisect skip 05fe125fa3237de2ec5bada80031e694de78909c
# skip: [edc90b7dc4ceef62ef0ad9cc6c3f5dc770e83ad2] KVM: MMU: fix SMAP virtualization
git bisect skip edc90b7dc4ceef62ef0ad9cc6c3f5dc770e83ad2
# skip: [910a6aae4e2e45855efc4a268e43eed2d8445575] KVM: MTRR: exactly define the size of variable MTRRs
git bisect skip 910a6aae4e2e45855efc4a268e43eed2d8445575
# skip: [822bf4833ecc8ea63c69f3ed894c13b4509c9e85] arm64: defconfig: enable memtest
git bisect skip 822bf4833ecc8ea63c69f3ed894c13b4509c9e85


-- 
Michael

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