Re: [Qemu-devel] [PATCH v19 3/9] pc: add a Virtual Machine Generation ID device

[Igor Mammedov <imammedo@redhat.com>]

On Tue, 16 Feb 2016 14:36:49 +0200
Marcel Apfelbaum <marcel@redhat.com> wrote:

> On 02/16/2016 02:17 PM, Igor Mammedov wrote:
> > On Tue, 16 Feb 2016 12:05:33 +0200
> > Marcel Apfelbaum <marcel@redhat.com> wrote:
> >  
> >> On 02/11/2016 06:30 PM, Michael S. Tsirkin wrote:  
> >>> On Thu, Feb 11, 2016 at 04:16:05PM +0100, Igor Mammedov wrote:  
> >>>> On Tue, 9 Feb 2016 14:17:44 +0200
> >>>> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> >>>>  
> >>>>> On Tue, Feb 09, 2016 at 11:46:08AM +0100, Igor Mammedov wrote:  
> >>>>>>> So the linker interface solves this rather neatly:
> >>>>>>> bios allocates memory, bios passes memory map to guest.
> >>>>>>> Served us well for several years without need for extensions,
> >>>>>>> and it does solve the VM GEN ID problem, even though
> >>>>>>> 1. it was never designed for huge areas like nvdimm seems to want to use
> >>>>>>> 2. we might want to add a new 64 bit flag to avoid touching low memory  
> >>>>>> linker interface is fine for some readonly data, like ACPI tables
> >>>>>> especially fixed tables not so for AML ones is one wants to patch it.
> >>>>>>
> >>>>>> However now when you want to use it for other purposes you start
> >>>>>> adding extensions and other guest->QEMU channels to communicate
> >>>>>> patching info back.
> >>>>>> It steals guest's memory which is also not nice and doesn't scale well.  
> >>>>>
> >>>>> This is an argument I don't get. memory is memory. call it guest memory
> >>>>> or RAM backed PCI BAR - same thing. MMIO is cheaper of course
> >>>>> but much slower.
> >>>>>
> >>>>> ...  
> >>>> It however matters for user, he pays for guest with XXX RAM but gets less
> >>>> than that. And that will be getting worse as a number of such devices
> >>>> increases.
> >>>>  
> >>>>>>> OK fine, but returning PCI BAR address to guest is wrong.
> >>>>>>> How about reading it from ACPI then? Is it really
> >>>>>>> broken unless there's *also* a driver?  
> >>>>>> I don't get question, MS Spec requires address (ADDR method),
> >>>>>> and it's read by ACPI (AML).  
> >>>>>
> >>>>> You were unhappy about DMA into guest memory.
> >>>>> As a replacement for DMA, we could have AML read from
> >>>>> e.g. PCI and write into RAM.
> >>>>> This way we don't need to pass address to QEMU.  
> >>>> That sounds better as it saves us from allocation of IO port
> >>>> and QEMU don't need to write into guest memory, the only question is
> >>>> if PCI_Config opregion would work with driver-less PCI device.  
> >>>
> >>> Or PCI BAR for that reason. I don't know for sure.
> >>>  
> >>>>
> >>>> And it's still pretty much not test-able since it would require
> >>>> fully running OSPM to execute AML side.  
> >>>
> >>> AML is not testable, but that's nothing new.
> >>> You can test reading from PCI.
> >>>  
> >>>>>  
> >>>>>> As for working PCI_Config OpRegion without driver, I haven't tried,
> >>>>>> but I wouldn't be surprised if it doesn't, taking in account that
> >>>>>> MS introduced _DSM doesn't.
> >>>>>>  
> >>>>>>>
> >>>>>>>  
> >>>>>>>>>>      Just compare with a graphics card design, where on device memory
> >>>>>>>>>>      is mapped directly at some GPA not wasting RAM that guest could
> >>>>>>>>>>      use for other tasks.  
> >>>>>>>>>
> >>>>>>>>> This might have been true 20 years ago.  Most modern cards do DMA.  
> >>>>>>>>
> >>>>>>>> Modern cards, with it's own RAM, map its VRAM in address space directly
> >>>>>>>> and allow users use it (GEM API). So they do not waste conventional RAM.
> >>>>>>>> For example NVIDIA VRAM is mapped as PCI BARs the same way like in this
> >>>>>>>> series (even PCI class id is the same)  
> >>>>>>>
> >>>>>>> Don't know enough about graphics really, I'm not sure how these are
> >>>>>>> relevant.  NICs and disks certainly do DMA.  And virtio gl seems to
> >>>>>>> mostly use guest RAM, not on card RAM.
> >>>>>>>  
> >>>>>>>>>>      VMGENID and NVDIMM use-cases look to me exactly the same, i.e.
> >>>>>>>>>>      instead of consuming guest's RAM they should be mapped at
> >>>>>>>>>>      some GPA and their memory accessed directly.  
> >>>>>>>>>
> >>>>>>>>> VMGENID is tied to a spec that rather arbitrarily asks for a fixed
> >>>>>>>>> address. This breaks the straight-forward approach of using a
> >>>>>>>>> rebalanceable PCI BAR.  
> >>>>>>>>
> >>>>>>>> For PCI rebalance to work on Windows, one has to provide working PCI driver
> >>>>>>>> otherwise OS will ignore it when rebalancing happens and
> >>>>>>>> might map something else over ignored BAR.  
> >>>>>>>
> >>>>>>> Does it disable the BAR then? Or just move it elsewhere?  
> >>>>>> it doesn't, it just blindly ignores BARs existence and maps BAR of
> >>>>>> another device with driver over it.  
> >>>>>
> >>>>> Interesting. On classical PCI this is a forbidden configuration.
> >>>>> Maybe we do something that confuses windows?
> >>>>> Could you tell me how to reproduce this behaviour?  
> >>>> #cat > t << EOF
> >>>> pci_update_mappings_del
> >>>> pci_update_mappings_add
> >>>> EOF
> >>>>
> >>>> #./x86_64-softmmu/qemu-system-x86_64 -snapshot -enable-kvm -snapshot \
> >>>>    -monitor unix:/tmp/m,server,nowait -device pci-bridge,chassis_nr=1 \
> >>>>    -boot menu=on -m 4G -trace events=t ws2012r2x64dc.img \
> >>>>    -device ivshmem,id=foo,size=2M,shm,bus=pci.1,addr=01
> >>>>
> >>>> wait till OS boots, note BARs programmed for ivshmem
> >>>>    in my case it was
> >>>>      01:01.0 0,0xfe800000+0x100
> >>>> then execute script and watch pci_update_mappings* trace events
> >>>>
> >>>> # for i in $(seq 3 18); do printf -- "device_add e1000,bus=pci.1,addr=%x\n" $i | nc -U /tmp/m; sleep 5; done;
> >>>>
> >>>> hotplugging e1000,bus=pci.1,addr=12 triggers rebalancing where
> >>>> Windows unmaps all BARs of nics on bridge but doesn't touch ivshmem
> >>>> and then programs new BARs, where:
> >>>>     pci_update_mappings_add d=0x7fa02ff0cf90 01:11.0 0,0xfe800000+0x20000
> >>>> creates overlapping BAR with ivshmem  
> >>>
> >>>
> >>> Thanks!
> >>> We need to figure this out because currently this does not
> >>> work properly (or maybe it works, but merely by chance).
> >>> Me and Marcel will play with this.
> >>>  
> >>
> >> I checked and indeed we have 2 separate problems:
> >>
> >> 1. ivshmem is declared as PCI RAM controller and Windows *does* have the drivers
> >>      for it, however it is not remapped on re-balancing.  
> > Does it really have a driver, i.e ivshmem specific one?
> > It should have its own driver or otherwise userspace
> > won't be able to access/work with it and it would be pointless
> > to add such device to machine.  
> 
> No, it does not.
so it's "PCI RAM controller", which is marked as NODRV in INF file,
NODRV they use as a stub to prevent Windows asking for driver assuming
that HW owns/manages device.
And when rebalancing happens Windows completely ignores NODRV
BARs which causes overlapping with devices that have PCI drivers.

> 
> >  
> >>      You can see on Device Manage 2 working devices with the same MMIO region - strange!
> >>      This may be because PCI RAM controllers can't be re-mapped? Even then, it should not be overridden.
> >>      Maybe we need to add a clue to the OS in ACPI regarding this range?
> >>
> >> 2. PCI devices with no driver installed are not re-mapped. This can be OK
> >>      from the Windows point of view because Resources Window does not show the MMIO range
> >>      for this device.
> >>
> >>      If the other (re-mapped) device is working, is pure luck. Both Memory Regions occupy the same range
> >>      and have the same priority.
> >>
> >> We need to think about how to solve this.
> >> One way would be to defer the BAR activation to the guest OS, but I am not sure of the consequences.  
> > deferring won't solve problem as rebalancing could happen later
> > and make BARs overlap.  
> 
> Why not? If we do not activate the BAR in firmware and Windows does not have a driver
> for it, will not activate it at all, right?
> Why would Windows activate the device BAR if it can't use it? At least this is what I hope.
> Any other idea would be appreciated.
> 
> 
> > I've noticed that at startup Windows unmaps and then maps BARs
> > at the same addresses where BIOS've put them before.  
> 
> Including devices without a working driver?
I've just tried, it does so for ivshmem.

> 
> 
> Thanks,
> Marcel
> 
> >  
> >> And this does not solve the ivshmem problem.  
> > So far the only way to avoid overlapping BARs due to Windows
> > doing rebalancing for driver-less devices is to pin such
> > BARs statically with _CRS in ACPI table but as Michael said
> > it fragments PCI address-space.
> >  
> >>
> >> Thanks,
> >> Marcel
> >>
> >>
> >>
> >>
> >>  
> >  
> 
>