[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@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
On Thu, Aug 17, 2017 at 02:24:14PM +0200, Andrea Bolognani wrote:
Address some minor flaws in the original document that were pointed out during review.
Signed-off-by: Andrea Bolognani <abologna@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
Specialized use case doesn't sound right to my ears, especially because use case describes and action/situation/process and in doing that it should be very clear ==> "specific", but I might be wrong of course and you indeed need a specific set of skills to cope with it :).
+ 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.
s/you/your
+ </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
I think you should use either: ... to a guest *and* at the same time you're also adding... or: ... at the same time *as* adding PCI devices...
+ 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
^right you are (w/r/t specialized controller). These were just stylistic nitpicks I noted, as you've addressed the notes from the previous review in a very clear manner, so no issue there from my perspective. ACK Erik
On Fri, 2017-08-18 at 15:38 +0200, Erik Skultety wrote:
These were just stylistic nitpicks I noted, as you've addressed the notes from the previous review in a very clear manner, so no issue there from my perspective.
ACK
Pushed now. Thanks for the review :) -- Andrea Bolognani / Red Hat / Virtualization
participants (2)
-
Andrea Bolognani -
Erik Skultety