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

On Mon, Aug 10, 2015 at 12:08:02PM -0400, Laine Stump wrote: > On 08/10/2015 11:09 AM, Daniel P. Berrange wrote: > > 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(a)samsung.com&gt; > >>>> --- > >>>> 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. > > Except that auto-address assignment happens after the parse, but before > we write the config to disk (i.e. we don't wait until the domain is > started. I'm guessing that was originally done because we didn't want to > be potentially re-writing the config to disk every time the domain was > started.) Also, we can't assume that "no PCI devices used at start" > means "no PCI devices will ever be desired" because hotplug. Ok, so it sounds to me like we are pretty much forced in to a position where we have to probe QEMU capabilities at XML define time. If we aim to minimize the usage of capabilities at define time, this won't be the end of the world. The only negative impact it would have is that if someone has old QEMU installed at define time and later upgrades to a QEMU with PCI support before starting the guest, they'll lack the PCI controller. THat's niche enough scenario that I think we can live with it. So from that POV, maybe we should just clean up the current merged patch so that it is properly exercised in the test suite by providing capabilities