From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from eggs.gnu.org ([2001:4830:134:3::10]:48146) by lists.gnu.org with esmtp (Exim 4.71) (envelope-from ) id 1cdcbd-0003pu-LS for qemu-devel@nongnu.org; Tue, 14 Feb 2017 07:53:20 -0500 Received: from Debian-exim by eggs.gnu.org with spam-scanned (Exim 4.71) (envelope-from ) id 1cdcbZ-0007xd-Oi for qemu-devel@nongnu.org; Tue, 14 Feb 2017 07:53:17 -0500 References: <20170208061602.17666-1-david@gibson.dropbear.id.au> <46d64512-1085-6d22-1e0f-660757ff7131@redhat.com> <20170209041634.GC14524@umbus> <95706652-0a80-92fc-951b-7a454d496ddf@redhat.com> <20170210003746.GP27610@umbus.fritz.box> <5ea3785c-b979-8b8c-3ab0-243d69384697@redhat.com> <20170213043307.GT25381@umbus> <45446029-e404-77d4-754e-5541a506bb7c@redhat.com> <20170214041532.GF2169@umbus.fritz.box> From: Marcel Apfelbaum Message-ID: Date: Tue, 14 Feb 2017 14:53:08 +0200 MIME-Version: 1.0 In-Reply-To: <20170214041532.GF2169@umbus.fritz.box> Content-Type: text/plain; charset=windows-1252; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Qemu-devel] [RFC] virtio-pci: Allow PCIe virtio devices on root bus List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: David Gibson Cc: Laszlo Ersek , abologna@redhat.com, lvivier@redhat.com, thuth@redhat.com, mst@redhat.com, qemu-devel@nongnu.org, qemu-ppc@nongnu.org, kraxel@redhat.com, Eduardo Habkost mst@redhat.com On 02/14/2017 06:15 AM, David Gibson wrote: > On Mon, Feb 13, 2017 at 12:14:23PM +0200, Marcel Apfelbaum wrote: >> On 02/13/2017 06:33 AM, David Gibson wrote: >>> On Sun, Feb 12, 2017 at 09:05:46PM +0200, Marcel Apfelbaum wrote: >>>> On 02/10/2017 02:37 AM, David Gibson wrote: >>>>> On Thu, Feb 09, 2017 at 10:04:47AM +0100, Laszlo Ersek wrote: >>>>>> On 02/09/17 05:16, David Gibson wrote: >>>>>>> On Wed, Feb 08, 2017 at 11:40:50AM +0100, Laszlo Ersek wrote: >>>>>>>> On 02/08/17 07:16, David Gibson wrote: >>>>>>>>> Marcel, >>>>>>>>> >>>>>>>>> Your original patch adding PCIe support to virtio-pci.c has the >>>>>>>>> limitation noted below that PCIe won't be enabled if the device is on >>>>>>>>> the root bus (rather than under a root or downstream port). As >>>>>>>>> reasoned below, I think removing the check is correct, even for x86 >>>>>>>>> (though it would rarely be useful there). But I could well have >>>>>>>>> missed something. Let me know if so... >>>>>>>>> >>>>>>>>> >>>>>>>>> >>>>>>>>> Virtio devices can appear as either vanilla PCI or PCI-Express devices >>>>>>>>> depending on the bus they're connected to. At the moment it will only >>>>>>>>> appear as vanilla PCI if connected to the root bus of a PCIe host bridge. >>>>>>>>> >>>>>>>>> Presumably this is to reflect the fact that PCIe devices usually need to >>>>>>>>> be connected to a root (or further downstream) port rather than directly >>>>>>>>> on the root bus. However, due to the odd requirements of the PAPR spec on the 'pseries' >>>>>>>>> machine type, it's normal for PCIe devices to appear on the root bus >>>>>>>>> without root ports. >>>>>>>>> >>>>>>>>> Further, even on x86, there's no inherent reason we couldn't present a >>>>>>>>> virtio device as an "integrated device" (typically used for things built >>>>>>>>> into the PCI chipset), and those devices *do* typically appear on the root >>>>>>>>> bus. >>>>>>>> >>>>>>>> I'm not personally making a counter-argument, just qouting some of >>>>>>>> the relevant parts of "docs/pcie.txt" ("PCI EXPRESS GUIDELINES"): >>>>>>> >>>>>>> So, an earlier discussion more or less concluded that the PCIe >>>>>>> guidelines don't really work with PAPR guests. That comes because >>>>>>> PAPR was designed with PowerVM in mind which allows PCI passthrough >>>>>>> but doesn't do any emulated PCI devices. So they wanted to present >>>>>>> passed through devices (virtual or phyical) to the guest without >>>>>>> inserting virtual root ports. >>>>>>> >>>>>>> Now, you can argue that this was a silly decision in PAPR, and you >>>>>>> could well be right, but there it is. >>>>>> >>>>>> I can totally accept this, but then we should state it as a fact near >>>>>> the top of "docs/pcie.txt". >>>>>> >>>>>>> >>>>>>>>> Place only the following kinds of devices directly on the Root Complex: >>>>>>>>> (1) PCI Devices (e.g. network card, graphics card, IDE controller), >>>>>>>>> not controllers. Place only legacy PCI devices on >>>>>>>>> the Root Complex. These will be considered Integrated Endpoints. >>>>>>>>> Note: Integrated Endpoints are not hot-pluggable. >>>>>>>>> >>>>>>>>> Although the PCI Express spec does not forbid PCI Express devices as >>>>>>>>> Integrated Endpoints, existing hardware mostly integrates legacy PCI >>>>>>>>> devices with the Root Complex. >>>>>>> >>>>>>> "Mostly".. on my laptop at least the GPU shows up as an integrated PCI >>>>>>> Express endpoint, so it's certainly not the case that *all* root bus >>>>>>> devices are legacy. >>>>>>> >>>>>>>> Guest OSes are suspected to behave >>>>>>>>> strangely when PCI Express devices are integrated >>>>>>>>> with the Root Complex. >>>>>>> >>>>>>> Clearly not that strangely, that often, since my laptop works just fine. >>>>>>> >>>>>>>>> >>>>>>>>> [...] >>>>>>>>> >>>>>>>>> 2.2 PCI Express only hierarchy >>>>>>>>> ============================== >>>>>>>>> Always use PCI Express Root Ports to start PCI Express hierarchies. >>>>>>>> >>>>>>>> Above you mention "it's normal for PCIe devices to appear on the root bus without root ports". >>>>>>> >>>>>>> Well "normal" perhaps wasn't the right word. Let's say precedented, >>>>>>> if uncommon. >>>>>>> >>>>>>>> Let me turn the question around: is it a *problem* for "pseries" if >>>>>>>> we require root ports? If so, why exactly? >>>>>>> >>>>>>> That's.. a complex question. At least Linux guests (and we don't >>>>>>> support any others yet) might cope with the addition of root ports. >>>>>>> Maybe. I have discussed this option with BenH and others. >>>>>>> >>>>>>> However it is gratuitously different from how PCIe devices will >>>>>>> typically appear for the same guest running under PowerVM. Although I >>>>>>> suspect Linux would cope with the "normal standard" rather than "PAPR >>>>>>> standard" presentation, I'm not as confident about it as I would like. >>>>>>> >>>>>>> Another consideration here is that other PCIe capable qemu emulated >>>>>>> devices, such as XHCI, will present fine as PCIe integrated endpoints >>>>>>> when attached to the root bus. Libvirt won't do that usually, of >>>>>>> course, and it may not be the recommended way of doing things (on PC) >>>>>>> but it's possible. I don't see any particular reason that virtio-pci >>>>>>> should enforce the root port requirement more so than any other >>>>>>> device. >>>>>>> >>>>>>>> On 02/08/17 07:16, David Gibson wrote: >>>>>>>>> >>>>>>>>> pcie_endpoint_cap_init() already automatically adjusts to advertise as >>>>>>>>> an integrated device rather than a "normal" PCIe endpoint when attached to >>>>>>>>> a root bus. So we can remove the check for root bus within virtio and >>>>>>>>> allow (at the user's discretion) a PCIe virtio bus to be attached to a >>>>>>>>> root bus. >>>>>>>> >>>>>>>> If Marcel thinks this is a good change, then I think we should go >>>>>>>> through "docs/pcie.txt" with a fine-toothed comb, and update all >>>>>>>> relevant spots. (If Marcel agrees, perhaps you can include such >>>>>>>> hunks in your patch at once.) >>>>>>> >>>>>>> Actually, I think that would be a neverending process. Maybe better >>>>>>> to put in a whole different spapr-pcie.txt with the assorted ways that >>>>>>> PAPR violates PCIe conventions. >>>>>> >>>>>> That works for me too, but I think it would be a lot more work for you >>>>>> and others. >>>>>> >>>>>> I plan on consulting "docs/pcie.txt" frequently; among other things, for >>>>>> deciding debates. Thus, improving the scope of "docs/pcie.txt" is very >>>>>> welcome IMO. >>>>>> >>>>>>> >>>>>>>> It also may have consequences for libvirt (but I see you addressed >>>>>>>> Andrea at once, which is great). >>>>>>> >>>>>>> Right, I've been discussed this with Andrea all along. We're working >>>>>>> on a proposed PAPR specific way of allocating PCI and PCIe addresses >>>>>>> (different from the PCIe normal way, but the same as each other). >>>>>>> That will simplify adding PCIe support to PAPR, and also has some >>>>>>> other advantages for PAPR guests (related to the platform specific >>>>>>> isolation, hotplug and error recovery mechanisms - also different >>>>>>> from the normal PCIe ones). >>>>>> >>>>>> Great, if Andrea is aware, that's a relief. >>>>>> >>>>>> Can you resubmit this patch with a small hunk for "docs/pcie.txt" that >>>>>> removes PAPR from the scope? >>>>> >>>> >>>> Hi David, >>>> Sorry for the delay, I just came back from PTO. >>>> >>>>> Well, first I'd like to see if Marcel knows of some reason I didn't >>>>> think of why this test is important for virtio particularly here. But >>>>> assuming the basic idea is acceptable, then yes, I'll update pcie.txt. >>>>> >>>> >>>> There are two reasons for keeping virtio Integrated Endpoints as PCI devices. >>>> 1. The first point is generic; even if having PCIe devices as Integrated Endpoints should be OK, >>>> is not recommended because some guests may miss-behave (*). X86 arch supports a large number >>>> of guests and we don't want to check and fix everything if *we don't have to*. >>>> Even if is not written anywhere and there are actually some PCI Express Integrated Endpoints, >>>> most of them are legacy PCI devices (I actually think this is why we have Integrated Endpoints >>>> at all, but I might be wrong). >>> >>> Hm, ok. Could we implement that restriction in the pci/pcie core >>> rather than in the virtio device? >> >> I am not sure if we should do that. Most of the devices are PCI or PCIe. >> Only some devices are "hybrid", the virtio devices, XHCI and I am not sure we have more. > > Ok, I see your point, the pcie core might not be right. But it still > seems really weird to have it explicitly in each hybrid device, even > if it's just the two. As the code stands right now, XHCI and virtio > have different behaviour, without a clear reason for it. > I suppose XHCI can behave the same as virtio if Gerd has nothing against it (remain PCI if plugged into the Root Complex), but I don't know how it will help your case. > Maybe a hybrid_setup() helper function? > How would it help PAPR scenario? Anyway, Eduardo is working on supplying new query interfaces for libvirt and he touches this subject, I think he does plan to mark the hybrid devices in some way. >>> That would then protect things like >>> XHCI as well. >> >> I don't see a reason to have XHCI as Integrated Endpoint, I think it should be always >> plugged into a root port. (for x86. arm and power) > > No, not for Power. Well, ok, yes for most ppc machine types, but not > for the paravirtualized 'pseries' machine (which is the one we most > care about). > > Well.. I guess it doesn't need to be an Integrated Endpoint per se, > but we should be able to have it appear on the guest root bus. > > The thing to realize is that the paravirtualized PCI interfaces in > PAPR mean there's very little guest visible difference between PCI and > PCIe. In fact in most ways you could say that the paravirtualized bus > operates like a vanilla PCI bus.. except that it does provide a way to > access PCIe extended config space. Maybe we can have a new bus type deriving from PCIBus and sibling of the PCIe Bus. Then we can have the same rules as the PCI bus and add tweaks when necessary. This does not solve the virtio problem since the code is actively looking for a PCIe Root Port and we don't have it for PAPR. Which means that you can use it to > drive PCIe devices just fine. "Bus level" PCIe extensions like AER > and PCIe standard hotplug won't work, but PAPR has its own mechanisms > for those (common between PCI and PCIe). > > I did float the idea of having the pseries PCI bus remain plain PCI > but with a special flag to allow PCIe devices to be attached to it > anyway. It wasn't greeted with much enthusiasm.. > Can you point me to the discussion please? It seems similar to what I proposed above. As you properly described it, is much closer to PCI then PCIe, even the only characteristic that makes it "a little" PCIe, the Extended Configuration Space support, is done with an alternative interface. I agree the PAPR bus is not PCIe. >> And for my purposes it would also make it easier to >>> implement aa machine type specific hook to re-allow that configuration >>> on pseries. >> >> I agree we need a solution for PAPR. >> >> What about a pcie_papr() function and then: >> >> diff --git a/hw/virtio/virtio-pci.c b/hw/virtio/virtio-pci.c >> index 5ce42af..2c646ae 100644 >> --- a/hw/virtio/virtio-pci.c >> +++ b/hw/virtio/virtio-pci.c >> @@ -1804,7 +1804,7 @@ static void virtio_pci_realize(PCIDevice *pci_dev, Error **errp) >> return; >> } >> >> - if (pcie_port && pci_is_express(pci_dev)) { >> + if ((pcie_port || pcie_parp()) && pci_is_express(pci_dev)) { >> int pos; >> >> pos = pcie_endpoint_cap_init(pci_dev, 0); > > That would be sufficient, yes, so I'll take it if we don't come to any > other solutions. OK It still seems weird to me to have this logic within > a specific device implementation though. > I suppose you have a point, let's wait for Gerd and maybe Michael to comment on that, but anyway it will not help your case. (it will only make XHCI PCI if plugged into Root Complex) >>> >>> At the moment XHCI and virtio-pci behave differently, which seems less >>> than ideal. >>> >>>> 2. The second point is virtio specific. Not all the guests have virtio 1.0 support (e.g RHEL 6) and we allow them >>>> to use legacy virtio devices as Integrated Endpoints (following the thought that this is why we have Integrated Endpoints) >>>> Making the virtio devices PCI Express, but not virtio 1.0 is also problematic since now we will have too much >>>> types of virtio devices. We want to keep it simple: virtio legacy >>>> -> PCI, virtio modern -> PCIe. >>> >>> Ok.. it's not obvious to me why integrated endpoint vs. under a root >>> port is relevant to this. Can't we enable/disable PCIe mode based >>> directly on the legacy/modern settings? >>> >> >> Yes, we can, but we don't want to do that. Previous setups will stop working >> and we will need libvirt to mange the disable-* properties. >> As a matter of fact the code today is after some discussions with libvirt guys. > > Uh.. I think I'm missing something.. but it's still not clear to me > why this would break existing setups or impose more work on libvirt. > Long story short, libvirt guys don't want to manually set the disable-* properties of virtio devices to make them comply to our goal (legacy -> PCI, modern PCIe). They also don't want to look at QEMU version/machine type to make a decision on the above properties. I agree the solution is not perfect, but at least it makes our testing matrix smaller and makes our PCIe guidelines a little easier to understand (e.g. all supported devices are PCIe, but if want legacy PCI devices put them on the Root Complex) XHCI being PCIe on Root Complex is an unintended exception, but we want it connected to a Root Port anyway, we don't have anything to gain from having it as Integrated Endpoint. We only loose a slot that can be used by 8 Root Ports assembled into one multi-function device. PAPR bus should not be considered PCIe and should have a different set of rules allowing PCIe devices to be plugged into Root Complex. Thanks, Marcel >>>> (*) A while ago Alex Williamson found such of issue, I think is this one: >>>> 0282ab (vfio/pci: Hide device PCIe capability on non-express buses for PCIe VMs) >>> >>> It's also not clear to me why this fix is relevant to the question. >>> That change disables the PCIe capability on a bus which is >>> not-express, but is under an express root bridge (and is therefore >>> clearly *not* on the root bus). For the case I'm talking about the >>> *is* on an express bus and it *is* the root bus. AFAICT that patch >>> would be relevant only for devices under a PCIe-to-PCI bridge on a >>> PCIe system. >>> >> >> I might have selected the wrong patch. > > Ok. > >> >> Thanks, >> Marcel >> >> >>>>>> be appreciated too, if that makes sense. (By default we aim at >>>>>> multi-arch / multi-target with this document; we may not have stated it >>>>>> explicitly, but AFAIR we intend to cover aarch64 / "virt" too.) >>>>> >>>>> Right, that was my understanding as well. >>>>> >>>> >>>> Indeed, we want the document to support them all. If PAPR is different, we should mention it. >>> >>> Sure. I'm trying to work out what we can/should do for pseries first; >>> I'll write something up for the docs when I have something I think is >>> ready to merge. >>> >> >