Messages in this thread | | | Subject | Re: [PATCH] xen-pciback: allow compiling on other archs than x86 | From | Juergen Gross <> | Date | Tue, 21 Sep 2021 08:49:42 +0200 |
| |
On 21.09.21 08:38, Oleksandr Andrushchenko wrote: > > On 21.09.21 09:07, Juergen Gross wrote: >> On 21.09.21 07:51, Oleksandr Andrushchenko wrote: >>> >>> On 21.09.21 08:20, Juergen Gross wrote: >>>> On 21.09.21 01:16, Stefano Stabellini wrote: >>>>> On Mon, 20 Sep 2021, Oleksandr Andrushchenko wrote: >>>>>> On 20.09.21 14:30, Juergen Gross wrote: >>>>>>> On 20.09.21 07:23, Oleksandr Andrushchenko wrote: >>>>>>>> Hello, Stefano! >>>>>>>> >>>>>>>> On 18.09.21 00:45, Stefano Stabellini wrote: >>>>>>>>> Hi Oleksandr, >>>>>>>>> >>>>>>>>> Why do you want to enable pciback on ARM? Is it only to "disable" a PCI >>>>>>>>> device in Dom0 so that it can be safely assigned to a DomU? >>>>>>>> Not only that >>>>>>>>> >>>>>>>>> I am asking because actually I don't think we want to enable the PV PCI >>>>>>>>> backend feature of pciback on ARM, right? That would clash with the PCI >>>>>>>>> assignment work you have been doing in Xen. They couldn't both work at >>>>>>>>> the same time. >>>>>>>> Correct, it is not used >>>>>>>>> >>>>>>>>> If we only need pciback to "park" a device in Dom0, wouldn't it be >>>>>>>>> possible and better to use pci-stub instead? >>>>>>>> >>>>>>>> Not only that, so pci-stub is not enough >>>>>>>> >>>>>>>> The functionality which is implemented by the pciback and the toolstack >>>>>>>> and which is relevant/missing/needed for ARM: >>>>>>>> >>>>>>>> 1. pciback is used as a database for assignable PCI devices, e.g. xl >>>>>>>> pci-assignable-{add|remove|list} manipulates that list. So, whenever the >>>>>>>> toolstack needs to know which PCI devices can be passed through it reads >>>>>>>> that from the relevant sysfs entries of the pciback. >>>>>>>> >>>>>>>> 2. pciback is used to hold the unbound PCI devices, e.g. when passing through >>>>>>>> a PCI device it needs to be unbound from the relevant device driver and bound >>>>>>>> to pciback (strictly speaking it is not required that the device is bound to >>>>>>>> pciback, but pciback is again used as a database of the passed through PCI >>>>>>>> devices, so we can re-bind the devices back to their original drivers when >>>>>>>> guest domain shuts down) >>>>>>>> >>>>>>>> 3. Device reset >>>>>>>> >>>>>>>> We have previously discussed on xen-devel ML possible solutions to that as from the >>>>>>>> above we see that pciback functionality is going to be only partially used on Arm. >>>>>>>> >>>>>>>> Please see [1] and [2]: >>>>>>>> >>>>>>>> 1. It is not acceptable to manage the assignable list in Xen itself >>>>>>>> >>>>>>>> 2. pciback can be split into two parts: PCI assignable/bind/reset handling and >>>>>>>> the rest like vPCI etc. >>>>>>>> >>>>>>>> 3. pcifront is not used on Arm >>>>>>> >>>>>>> It is neither in x86 PVH/HVM guests. >>>>>> Didn't know that, thank you for pointing >>>>>>> >>>>>>>> So, limited use of the pciback is one of the bricks used to enable PCI passthrough >>>>>>>> on Arm. It was enough to just re-structure the driver and have it run on Arm to achieve >>>>>>>> all the goals above. >>>>>>>> >>>>>>>> If we still think it is desirable to break the pciback driver into "common" and "pcifront specific" >>>>>>>> parts then it can be done, yet the patch is going to be the very first brick in that building. >>>>>>> >>>>>>> Doing this split should be done, as the pcifront specific part could be >>>>>>> omitted on x86, too, in case no PV guests using PCI passthrough have to >>>>>>> be supported. >>>>>> Agree, that the final solution should have the driver split >>>>>>> >>>>>>>> So, I think this patch is still going to be needed besides which direction we take. >>>>>>> >>>>>>> Some kind of this patch, yes. It might look different in case the split >>>>>>> is done first. >>>>>>> >>>>>>> I don't mind doing it in either sequence. >>>>>>> >>>>>> With this patch we have Arm on the same page as the above mentioned x86 guests, >>>>>> >>>>>> e.g. the driver has unused code, but yet allows Arm to function now. >>>>>> >>>>>> At this stage of PCI passthrough on Arm it is yet enough. Long term, when >>>>>> >>>>>> the driver gets split, Arm will benefit from that split too, but unfortunately I do not >>>>>> >>>>>> have enough bandwidth for that piece of work at the moment. >>>>> >>>>> That's fair and I don't want to scope-creep this simple patch asking for >>>>> an enormous rework. At the same time I don't think we should enable the >>>>> whole of pciback on ARM because it would be erroneous and confusing. >>> >>> As the first stage before the driver is split or ifdef's used - can we take the patch >>> as is now? In either way we chose this needs to be done, e.g. enable compiling >>> for other architectures and common code move. >> >> Fine with me in principle. I need to take a more thorough look >> at the patch, though. > Of course >> >>> >>>>> >>>>> I am wonder if there is a simple: >>>>> >>>>> if (!xen_pv_domain()) >>>>> return; >>>>> >>>>> That we could add in a couple of places in pciback to stop it from >>>>> initializing the parts we don't care about. Something along these lines >>>>> (untested and probably incomplete). >>>>> >>>>> What do you guys think? >>>> >>>> Uh no, not in this way, please. This will kill pci passthrough on x86 >>>> with dom0 running as PVH. I don't think this is working right now, but >>>> adding more code making it even harder to work should be avoided. >>>> >>>>> diff --git a/drivers/xen/xen-pciback/xenbus.c b/drivers/xen/xen-pciback/xenbus.c >>>>> index da34ce85dc88..991ba0a9b359 100644 >>>>> --- a/drivers/xen/xen-pciback/xenbus.c >>>>> +++ b/drivers/xen/xen-pciback/xenbus.c >>>>> @@ -15,6 +15,7 @@ >>>>> #include <xen/xenbus.h> >>>>> #include <xen/events.h> >>>>> #include <xen/pci.h> >>>>> +#include <xen/xen.h> >>>>> #include "pciback.h" >>>>> #define INVALID_EVTCHN_IRQ (-1) >>>>> @@ -685,8 +686,12 @@ static int xen_pcibk_xenbus_probe(struct xenbus_device *dev, >>>>> const struct xenbus_device_id *id) >>>>> { >>>>> int err = 0; >>>>> - struct xen_pcibk_device *pdev = alloc_pdev(dev); >>>>> + struct xen_pcibk_device *pdev; >>>>> + >>>>> + if (!xen_pv_domain()) >>>>> + return 0; >>>>> + pdev = alloc_pdev(dev); >>>> >>>> This hunk isn't needed, as with bailing out of xen_pcibk_xenbus_register >>>> early will result in xen_pcibk_xenbus_probe never being called. >>>> >>>>> if (pdev == NULL) { >>>>> err = -ENOMEM; >>>>> xenbus_dev_fatal(dev, err, >>>>> @@ -743,6 +748,9 @@ const struct xen_pcibk_backend *__read_mostly xen_pcibk_backend; >>>>> int __init xen_pcibk_xenbus_register(void) >>>>> { >>>>> + if (!xen_pv_domain()) >>>>> + return 0; >>>>> + >>>> >>>> Use #ifdef CONFIG_X86 instead. >>> >>> The title of this patch says that we want to allow this driver for other archs >>> and now we want to introduce "#ifdef CONFIG_X86" which doesn't sound >>> right with that respect. Instead, we may want having something like a >>> dedicated gate for this, e.g. "#ifdef CONFIG_XEN_PCIDEV_BACKEND_SUPP_PV" >>> or something which is architecture agnostic. >> >> Something like that, yes. But I'd rather use CONFIG_XEN_PCIDEV_BACKEND >> acting as this gate and introduce CONFIG_XEN_PCI_STUB for the stub >> functionality needed on Arm. XEN_PCIDEV_BACKEND would depend on X86 and >> select XEN_PCI_STUB, while on Arm XEN_PCI_STUB could be configured if >> wanted. The splitting of the driver can still be done later. > > Hm, pciback is now compiled when CONFIG_XEN_PCIDEV_BACKEND is enabled > and we want to skip some parts of its code when CONFIG_XEN_PCI_STUB is set. > So, I imagine that for x86 we just enable CONFIG_XEN_PCIDEV_BACKEND and the > driver compiles in its current state. For Arm we enable both CONFIG_XEN_PCIDEV_BACKEND > and CONFIG_XEN_PCI_STUB, so part of the driver is not compiled.
No, I'd rather switch to compiling xen-pciback when CONFIG_XEN_PCI_STUB is set and compile only parts of it when CONFIG_XEN_PCIDEV_BACKEND is not set (this will be the case on Arm).
This is another step in the right direction preparing the split.
But as said before, this is not a requirement by me to take your patch.
Juergen [unhandled content-type:application/pgp-keys][unhandled content-type:application/pgp-signature] | |