Re-tested this issue, For linux guest, fail to boot guest when attach more than 4 devices to the same switch. For windows guest, guest will boot fail when more than 3.  this is new qemu-kvm command line.

/usr/libexec/qemu-kvm -name 'virtio-network' -nodefaults -m 4G -smp 4,cores=2,threads=2,sockets=1 -M q35 -cpu Opteron_G2 -rtc base=utc,clock=host,driftfix=slew -k en-us -boot menu=on -monitor stdio -vnc :1 -spice disable-ticketing,port=5931 -qmp tcp:0:5555,server,nowait -vga qxl -device ioh3420,bus=pcie.0,id=root.0 -device x3130-upstream,bus=root.0,id=upstream -device xio3130-downstream,bus=upstream,id=downstream0,chassis=1 -device x3130-upstream,bus=downstream0,id=upstream1 -device xio3130-downstream,bus=upstream1,id=downstream1,chassis=2 -device xio3130-downstream,bus=upstream1,id=downstream2,chassis=3 -device xio3130-downstream,bus=upstream1,id=downstream3,chassis=4 -device xio3130-downstream,bus=upstream1,id=downstream4,chassis=5 -device xio3130-downstream,bus=upstream1,id=downstream5,chassis=6 -device xio3130-downstream,bus=upstream1,id=downstream6,chassis=7 

-drive file=/home/guest-rhel7.0-64.qcow3,if=none,id=drive-scsi-disk,format=qcow2,cache=none,werror=stop,rerror=stop -device virtio-scsi-pci,id=scsi0,bus=downstream1 -device scsi-disk,drive=drive-scsi-disk,bus=scsi0.0,scsi-id=0,lun=0,id=scsi-disk,bootindex=1 

-device virtio-net-pci,netdev=fuxc,mac=00:24:21:7f:0d:10,id=n1,bus=downstream2 -netdev tap,id=fuxc,vhost=on,script=/etc/qemu-ifup 

-device virtio-net-pci,netdev=fuxc1,mac=00:24:21:7f:0d:11,id=n2,bus=downstream3 -netdev tap,id=fuxc1,vhost=on,script=/etc/qemu-ifup 

-device virtio-net-pci,netdev=fuxc2,mac=00:24:21:7f:0d:12,id=n3,bus=downstream4 -netdev tap,id=fuxc2,vhost=on,script=/etc/qemu-ifup 

-device virtio-net-pci,netdev=fuxc3,mac=00:24:21:7f:0d:13,id=n4,bus=downstream5 -netdev tap,id=fuxc3,vhost=on,script=/etc/qemu-ifup

-device sga -chardev socket,id=serial0,path=/var/test1,server,nowait -device isa-serial,chardev=serial0

Created attachment 777275 [details]
guest screenshot

could this be re-tested on a recent kernel please?
also, does this happen with piix and not q35?

Please re-test with recent kernel and qemu packages.  Attempt to reproduce this
on the following environment failed.

Kernel: 3.10.0-71.el7.x86_64
qemu: qemu-kvm-1.5.3-37.el7.x86_64

Thanks
-vlad

Re-tested this bug with qemu-kvm-1.5.3-37.el7.x86_64 and 3.10.0-73.el7.x86_64.

Test steps as comment2.

result: fail to boot guest when attach more than 4 devices to the same switch.

so, This bug still can be reproduced with the latest qemu-kvm and kernel.

This only happens with q35.  This also only happens when you have more then 5 devices each hanging off its won pcie down port.  In the reproducer from
Comment 2, we have 5 devices, each using bus downstream1-5.  The fact that
some of them are virtio_net and others are virtio_scsi doesn't seem to
matter.  The scsi device isn't detected either.

Interestingly, connecting them all to a single downstream bus makes things work
again. Even distributing them to 4 downstream buses still works.  It's when we
hit downstream5 is where we have a problem.

After looking at what iPXE does, it looks like it is trying to access the vq.
Attempting to trap in virtio_pci_config_write() succeeds when using a working
configuration and fails when using a non-working set-up.  It looks like when
there are 5 downstream buses being used, qemu doesn't correctly locate the virtio memory region.  Currently trying to debug this and determine where
the memory access ends up. It isn't virtio as its supposed to be.

-vlad

Verified that solution to Bug 1055832 also solves this issue. Changing component to seabios and marking as duplicate.

*** This bug has been marked as a duplicate of bug 1055832 ***