[libvirt] [PATCH] docs: Improve PCI topology and hotplug guidelines
Address some minor flaws in the original document that
were pointed out during review.
Signed-off-by: Andrea Bolognani <abologna(a)redhat.com>
---
docs/pci-hotplug.html.in | 46 ++++++++++++++++++++++++++++++++--------------
1 file changed, 32 insertions(+), 14 deletions(-)
diff --git a/docs/pci-hotplug.html.in b/docs/pci-hotplug.html.in
index 809e36f5d..a90d47bc3 100644
--- a/docs/pci-hotplug.html.in
+++ b/docs/pci-hotplug.html.in
@@ -13,10 +13,12 @@
<p>
The reason for this apparent limitation is the fact that each
hotplugged PCI device might require additional PCI controllers to
- be added to the guest, and libvirt has no way of knowing in advance
- how many devices will be hotplugged during the guest's lifetime,
- thus making it impossible to automatically provide the right amount
- of PCI controllers: any arbitrary number would end up being too big
+ be added to the guest. Since most PCI controllers can't be
+ hotplugged, they need to be added before the guest is started;
+ however, libvirt has no way of knowing in advance how many devices
+ will be hotplugged during the guest's lifetime, thus making it
+ impossible to automatically provide the right amount of PCI
+ controllers: any arbitrary number would end up being too big
for some users, and too small for others.
</p>
<p>
@@ -53,6 +55,14 @@
emulated or assigned from the host.
</p>
<p>
+ If you have a very specialized use case, such as the appliances
+ used by <a href="http://libguestfs.org/">libguestfs</a>
behind
+ the scenes to access disk images, and this automatically-added
+ <code>pcie-root-port</code> controller ends up being a nuisance,
+ you can prevent libvirt from adding it by manually managing PCI
+ controllers and addresses according to you needs.
+ </p>
+ <p>
Slots on the <code>pcie-root</code> controller do not support
hotplug, so the device will be hotplugged into the
<code>pcie-root-port</code> controller. If you plan to hotplug
@@ -73,6 +83,12 @@
remaining details automatically.
</p>
<p>
+ Note that if you're adding PCI controllers to a guest at the
+ same time you're also adding PCI devices, some of the
+ controllers will be used for the newly-added devices and won't
+ be available for hotplug once the guest has been started.
+ </p>
+ <p>
If you expect to hotplug legacy PCI devices, then you will need
specialized controllers, since all those mentioned above are
intended for PCI Express devices only: add
@@ -84,7 +100,8 @@
<p>
and you'll be able to hotplug up to 31 legacy PCI devices,
- either emulated or assigned from the host.
+ either emulated or assigned from the host, in the slots
+ from 0x01 to 0x1f of the <code>pci-bridge</code> controller.
</p>
<h3><a name="x86_64-i440fx">i440fx (pc) machine
type</a></h3>
@@ -98,9 +115,10 @@
<controller type='pci' index='0'
model='pci-root'/></pre>
<p>
- where each of the 31 slots on the <code>pci-root</code>
- controller is hotplug capable and can accept a legacy PCI
- device, either emulated or assigned from the guest.
+ where each of the 31 slots (from 0x01 to 0x1f) on the
+ <code>pci-root</code> controller is hotplug capable and
+ can accept a legacy PCI device, either emulated or
+ assigned from the guest.
</p>
<h2><a name="ppc64">ppc64 architecture</a></h2>
@@ -119,12 +137,12 @@
</controller></pre>
<p>
- The 31 slots on a <code>pci-root</code> controller are all
- hotplug capable and, despite the name suggesting otherwise,
- starting with QEMU 2.9 all of them can accept PCI Express
- devices in addition to legacy PCI devices; however,
- libvirt will only place emulated devices on the default
- <code>pci-root</code> controller.
+ The 31 slots, from 0x01 to 0x1f, on a <code>pci-root</code>
+ controller are all hotplug capable and, despite the name
+ suggesting otherwise, starting with QEMU 2.9 all of them
+ can accept PCI Express devices in addition to legacy PCI
+ devices; however, libvirt will only place emulated devices
+ on the default <code>pci-root</code> controller.
</p>
<p>
In order to take advantage of improved error reporting and
--
2.13.5