[libvirt] [PATCH v5 2/4] qemu: Add PCI-Express root to ARM virt machine

[Daniel P. Berrange]

On Thu, Aug 06, 2015 at 07:46:58PM +0200, Martin Kletzander wrote:
> On Thu, Aug 06, 2015 at 06:37:41PM +0200, Martin Kletzander wrote:
> >On Fri, Jul 17, 2015 at 02:27:45PM +0300, Pavel Fedin wrote:
> >>Here we assume that if qemu supports generic PCI host controller,
> >>it is a part of virt machine and can be used for adding PCI devices.
> >>
> >>In qemu this is actually a PCIe bus, so we also declare multibus
> >>capability so that 0'th bus is specified to qemu correctly as 'pcie.0'
> >>
> >>Signed-off-by: Pavel Fedin <p.fedin at samsung.com>
> >>---
> >>src/qemu/qemu_capabilities.c |  8 ++++++++
> >>src/qemu/qemu_domain.c       | 17 +++++++++++++----
> >>2 files changed, 21 insertions(+), 4 deletions(-)
> >>
> >>diff --git a/src/qemu/qemu_capabilities.c b/src/qemu/qemu_capabilities.c
> >>index d570fdd..f3486c7 100644
> >>--- a/src/qemu/qemu_capabilities.c
> >>+++ b/src/qemu/qemu_capabilities.c
> >>@@ -2138,6 +2138,14 @@ bool virQEMUCapsHasPCIMultiBus(virQEMUCapsPtr qemuCaps,
> >>       return false;
> >>   }
> >>
> >>+    if (ARCH_IS_ARM(def->os.arch)) {
> >>+        /* If 'virt' supports PCI, it supports multibus.
> >>+         * No extra conditions here for simplicity.
> >>+         */
> >
> >So every ARM qemu with the "virt" machine type supports both PCI and
> >multiqueue?  How about those "virt-*" for which you check below.  That
> >might not be related, I'm just curious.
> >
> >>+        if (STREQ(def->os.machine, "virt"))
> >>+            return true;
> >>+    }
> >>+
> >>   return false;
> >>}
> >>
> >>diff --git a/src/qemu/qemu_domain.c b/src/qemu/qemu_domain.c
> >>index 8b050a0..c7d14e4 100644
> >>--- a/src/qemu/qemu_domain.c
> >>+++ b/src/qemu/qemu_domain.c
> >>@@ -981,7 +981,7 @@ virDomainXMLNamespace virQEMUDriverDomainXMLNamespace = {
> >>static int
> >>qemuDomainDefPostParse(virDomainDefPtr def,
> >>                      virCapsPtr caps,
> >>-                       void *opaque ATTRIBUTE_UNUSED)
> >>+                       void *opaque)
> >>{
> >>   bool addDefaultUSB = true;
> >>   bool addImplicitSATA = false;
> >>@@ -1030,12 +1030,21 @@ qemuDomainDefPostParse(virDomainDefPtr def,
> >>       break;
> >>
> >>   case VIR_ARCH_ARMV7L:
> >>-       addDefaultUSB = false;
> >>-       addDefaultMemballoon = false;
> >>-       break;
> >>   case VIR_ARCH_AARCH64:
> >>      addDefaultUSB = false;
> >>      addDefaultMemballoon = false;
> >>+       if (STREQ(def->os.machine, "virt") ||
> >>+           STRPREFIX(def->os.machine, "virt-")) {
> >>+           virQEMUDriverPtr driver = opaque;
> >>+
> >>+           /* This condition is actually a (temporary) hack for test suite which
> >>+            * does not create capabilities cache */
> >
> >Few questions here.  a) how "temporary" is this since you're not
> >removing it in this series?  b) for what tests you need this hack and
> >what part of the below is the hack?
> >
> >Moreover, you cannot use capabilities when defining an XML.  The
> >emulator can change between the domain is defined and started, so you
> >cannot know with what emulator this will be started.
> >
> >I see Michal (Cc'd) just pushed this, I probably just missed the mail
> 
> Of course I forgot, Cc'ing now.

I agree with your core statement that we should not be using the QEMU
capabilities when defining the XML. With all existing scenarios we have
been able to determine whether to add the implicit PCI controller based
on the machine type name only, because with every other QEMU arch when
doing such a major change as adding a PCI bus, they have created a new
machine type.  The problem is that arm 'virt' machine type is not stable,
it is being changed arbitrarily in new QEMU releases :-(

So AFAIK, that leaves us with 3 choices

 - Never add PCI controller at time the XML is defined, on the basis
   that we have to be conservative in what we add to cope with old QEMU

 - Always add PCI controller at time the XML is defined, on the basis
   that most people will have new enough QEMU because ARM 'virt' machine
   type is very much still in development, so no one will serously stick
   with the older QEMU versions which lack PCI.

 - Use the capabilities in XML post-parse to conditionally add the
   PCI controller. This is what was currently merged

I don't think option 1 makes much sense as it'll harm ARM arch forever
more, to cope with QEMU versions that will almost never be used in
practice.

I'd be inclined to go with option 2, and then if any PCI devices are
actually used with the guest, check the capability at start time when
we are doing auto-address assignment.

Regards,
Daniel
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