Re: [Qemu-devel] [PULL 14/28] exec: make address spaces 64-bit wide

["Michael S. Tsirkin" <mst@redhat.com>]

On Tue, Jan 14, 2014 at 09:39:24AM -0700, Alex Williamson wrote:
> On Tue, 2014-01-14 at 18:18 +0200, Michael S. Tsirkin wrote:
> > On Tue, Jan 14, 2014 at 09:15:14AM -0700, Alex Williamson wrote:
> > > On Tue, 2014-01-14 at 18:03 +0200, Michael S. Tsirkin wrote:
> > > > On Tue, Jan 14, 2014 at 08:57:58AM -0700, Alex Williamson wrote:
> > > > > On Tue, 2014-01-14 at 14:07 +0200, Michael S. Tsirkin wrote:
> > > > > > On Mon, Jan 13, 2014 at 03:48:11PM -0700, Alex Williamson wrote:
> > > > > > > On Mon, 2014-01-13 at 22:48 +0100, Alexander Graf wrote:
> > > > > > > > 
> > > > > > > > > Am 13.01.2014 um 22:39 schrieb Alex Williamson <alex.williamson@redhat.com>:
> > > > > > > > > 
> > > > > > > > >> On Sun, 2014-01-12 at 16:03 +0100, Alexander Graf wrote:
> > > > > > > > >>> On 12.01.2014, at 08:54, Michael S. Tsirkin <mst@redhat.com> wrote:
> > > > > > > > >>> 
> > > > > > > > >>>> On Fri, Jan 10, 2014 at 08:31:36AM -0700, Alex Williamson wrote:
> > > > > > > > >>>>> On Fri, 2014-01-10 at 14:55 +0200, Michael S. Tsirkin wrote:
> > > > > > > > >>>>>> On Thu, Jan 09, 2014 at 03:42:22PM -0700, Alex Williamson wrote:
> > > > > > > > >>>>>>> On Thu, 2014-01-09 at 23:56 +0200, Michael S. Tsirkin wrote:
> > > > > > > > >>>>>>>> On Thu, Jan 09, 2014 at 12:03:26PM -0700, Alex Williamson wrote:
> > > > > > > > >>>>>>>>> On Thu, 2014-01-09 at 11:47 -0700, Alex Williamson wrote:
> > > > > > > > >>>>>>>>>> On Thu, 2014-01-09 at 20:00 +0200, Michael S. Tsirkin wrote:
> > > > > > > > >>>>>>>>>>> On Thu, Jan 09, 2014 at 10:24:47AM -0700, Alex Williamson wrote:
> > > > > > > > >>>>>>>>>>>> On Wed, 2013-12-11 at 20:30 +0200, Michael S. Tsirkin wrote:
> > > > > > > > >>>>>>>>>>>> From: Paolo Bonzini <pbonzini@redhat.com>
> > > > > > > > >>>>>>>>>>>> 
> > > > > > > > >>>>>>>>>>>> As an alternative to commit 818f86b (exec: limit system memory
> > > > > > > > >>>>>>>>>>>> size, 2013-11-04) let's just make all address spaces 64-bit wide.
> > > > > > > > >>>>>>>>>>>> This eliminates problems with phys_page_find ignoring bits above
> > > > > > > > >>>>>>>>>>>> TARGET_PHYS_ADDR_SPACE_BITS and address_space_translate_internal
> > > > > > > > >>>>>>>>>>>> consequently messing up the computations.
> > > > > > > > >>>>>>>>>>>> 
> > > > > > > > >>>>>>>>>>>> In Luiz's reported crash, at startup gdb attempts to read from address
> > > > > > > > >>>>>>>>>>>> 0xffffffffffffffe6 to 0xffffffffffffffff inclusive.  The region it gets
> > > > > > > > >>>>>>>>>>>> is the newly introduced master abort region, which is as big as the PCI
> > > > > > > > >>>>>>>>>>>> address space (see pci_bus_init).  Due to a typo that's only 2^63-1,
> > > > > > > > >>>>>>>>>>>> not 2^64.  But we get it anyway because phys_page_find ignores the upper
> > > > > > > > >>>>>>>>>>>> bits of the physical address.  In address_space_translate_internal then
> > > > > > > > >>>>>>>>>>>> 
> > > > > > > > >>>>>>>>>>>>   diff = int128_sub(section->mr->size, int128_make64(addr));
> > > > > > > > >>>>>>>>>>>>   *plen = int128_get64(int128_min(diff, int128_make64(*plen)));
> > > > > > > > >>>>>>>>>>>> 
> > > > > > > > >>>>>>>>>>>> diff becomes negative, and int128_get64 booms.
> > > > > > > > >>>>>>>>>>>> 
> > > > > > > > >>>>>>>>>>>> The size of the PCI address space region should be fixed anyway.
> > > > > > > > >>>>>>>>>>>> 
> > > > > > > > >>>>>>>>>>>> Reported-by: Luiz Capitulino <lcapitulino@redhat.com>
> > > > > > > > >>>>>>>>>>>> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
> > > > > > > > >>>>>>>>>>>> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
> > > > > > > > >>>>>>>>>>>> ---
> > > > > > > > >>>>>>>>>>>> exec.c | 8 ++------
> > > > > > > > >>>>>>>>>>>> 1 file changed, 2 insertions(+), 6 deletions(-)
> > > > > > > > >>>>>>>>>>>> 
> > > > > > > > >>>>>>>>>>>> diff --git a/exec.c b/exec.c
> > > > > > > > >>>>>>>>>>>> index 7e5ce93..f907f5f 100644
> > > > > > > > >>>>>>>>>>>> --- a/exec.c
> > > > > > > > >>>>>>>>>>>> +++ b/exec.c
> > > > > > > > >>>>>>>>>>>> @@ -94,7 +94,7 @@ struct PhysPageEntry {
> > > > > > > > >>>>>>>>>>>> #define PHYS_MAP_NODE_NIL (((uint32_t)~0) >> 6)
> > > > > > > > >>>>>>>>>>>> 
> > > > > > > > >>>>>>>>>>>> /* Size of the L2 (and L3, etc) page tables.  */
> > > > > > > > >>>>>>>>>>>> -#define ADDR_SPACE_BITS TARGET_PHYS_ADDR_SPACE_BITS
> > > > > > > > >>>>>>>>>>>> +#define ADDR_SPACE_BITS 64
> > > > > > > > >>>>>>>>>>>> 
> > > > > > > > >>>>>>>>>>>> #define P_L2_BITS 10
> > > > > > > > >>>>>>>>>>>> #define P_L2_SIZE (1 << P_L2_BITS)
> > > > > > > > >>>>>>>>>>>> @@ -1861,11 +1861,7 @@ static void memory_map_init(void)
> > > > > > > > >>>>>>>>>>>> {
> > > > > > > > >>>>>>>>>>>>    system_memory = g_malloc(sizeof(*system_memory));
> > > > > > > > >>>>>>>>>>>> 
> > > > > > > > >>>>>>>>>>>> -    assert(ADDR_SPACE_BITS <= 64);
> > > > > > > > >>>>>>>>>>>> -
> > > > > > > > >>>>>>>>>>>> -    memory_region_init(system_memory, NULL, "system",
> > > > > > > > >>>>>>>>>>>> -                       ADDR_SPACE_BITS == 64 ?
> > > > > > > > >>>>>>>>>>>> -                       UINT64_MAX : (0x1ULL << ADDR_SPACE_BITS));
> > > > > > > > >>>>>>>>>>>> +    memory_region_init(system_memory, NULL, "system", UINT64_MAX);
> > > > > > > > >>>>>>>>>>>>    address_space_init(&address_space_memory, system_memory, "memory");
> > > > > > > > >>>>>>>>>>>> 
> > > > > > > > >>>>>>>>>>>>    system_io = g_malloc(sizeof(*system_io));
> > > > > > > > >>>>>>>>>>> 
> > > > > > > > >>>>>>>>>>> This seems to have some unexpected consequences around sizing 64bit PCI
> > > > > > > > >>>>>>>>>>> BARs that I'm not sure how to handle.
> > > > > > > > >>>>>>>>>> 
> > > > > > > > >>>>>>>>>> BARs are often disabled during sizing. Maybe you
> > > > > > > > >>>>>>>>>> don't detect BAR being disabled?
> > > > > > > > >>>>>>>>> 
> > > > > > > > >>>>>>>>> See the trace below, the BARs are not disabled.  QEMU pci-core is doing
> > > > > > > > >>>>>>>>> the sizing an memory region updates for the BARs, vfio is just a
> > > > > > > > >>>>>>>>> pass-through here.
> > > > > > > > >>>>>>>> 
> > > > > > > > >>>>>>>> Sorry, not in the trace below, but yes the sizing seems to be happening
> > > > > > > > >>>>>>>> while I/O & memory are enabled int he command register.  Thanks,
> > > > > > > > >>>>>>>> 
> > > > > > > > >>>>>>>> Alex
> > > > > > > > >>>>>>> 
> > > > > > > > >>>>>>> OK then from QEMU POV this BAR value is not special at all.
> > > > > > > > >>>>>> 
> > > > > > > > >>>>>> Unfortunately
> > > > > > > > >>>>>> 
> > > > > > > > >>>>>>>>>>> After this patch I get vfio
> > > > > > > > >>>>>>>>>>> traces like this:
> > > > > > > > >>>>>>>>>>> 
> > > > > > > > >>>>>>>>>>> vfio: vfio_pci_read_config(0000:01:10.0, @0x10, len=0x4) febe0004
> > > > > > > > >>>>>>>>>>> (save lower 32bits of BAR)
> > > > > > > > >>>>>>>>>>> vfio: vfio_pci_write_config(0000:01:10.0, @0x10, 0xffffffff, len=0x4)
> > > > > > > > >>>>>>>>>>> (write mask to BAR)
> > > > > > > > >>>>>>>>>>> vfio: region_del febe0000 - febe3fff
> > > > > > > > >>>>>>>>>>> (memory region gets unmapped)
> > > > > > > > >>>>>>>>>>> vfio: vfio_pci_read_config(0000:01:10.0, @0x10, len=0x4) ffffc004
> > > > > > > > >>>>>>>>>>> (read size mask)
> > > > > > > > >>>>>>>>>>> vfio: vfio_pci_write_config(0000:01:10.0, @0x10, 0xfebe0004, len=0x4)
> > > > > > > > >>>>>>>>>>> (restore BAR)
> > > > > > > > >>>>>>>>>>> vfio: region_add febe0000 - febe3fff [0x7fcf3654d000]
> > > > > > > > >>>>>>>>>>> (memory region re-mapped)
> > > > > > > > >>>>>>>>>>> vfio: vfio_pci_read_config(0000:01:10.0, @0x14, len=0x4) 0
> > > > > > > > >>>>>>>>>>> (save upper 32bits of BAR)
> > > > > > > > >>>>>>>>>>> vfio: vfio_pci_write_config(0000:01:10.0, @0x14, 0xffffffff, len=0x4)
> > > > > > > > >>>>>>>>>>> (write mask to BAR)
> > > > > > > > >>>>>>>>>>> vfio: region_del febe0000 - febe3fff
> > > > > > > > >>>>>>>>>>> (memory region gets unmapped)
> > > > > > > > >>>>>>>>>>> vfio: region_add fffffffffebe0000 - fffffffffebe3fff [0x7fcf3654d000]
> > > > > > > > >>>>>>>>>>> (memory region gets re-mapped with new address)
> > > > > > > > >>>>>>>>>>> qemu-system-x86_64: vfio_dma_map(0x7fcf38861710, 0xfffffffffebe0000, 0x4000, 0x7fcf3654d000) = -14 (Bad address)
> > > > > > > > >>>>>>>>>>> (iommu barfs because it can only handle 48bit physical addresses)
> > > > > > > > >>>>>>>>>> 
> > > > > > > > >>>>>>>>>> Why are you trying to program BAR addresses for dma in the iommu?
> > > > > > > > >>>>>>>>> 
> > > > > > > > >>>>>>>>> Two reasons, first I can't tell the difference between RAM and MMIO.
> > > > > > > > >>>>>>> 
> > > > > > > > >>>>>>> Why can't you? Generally memory core let you find out easily.
> > > > > > > > >>>>>> 
> > > > > > > > >>>>>> My MemoryListener is setup for &address_space_memory and I then filter
> > > > > > > > >>>>>> out anything that's not memory_region_is_ram().  This still gets
> > > > > > > > >>>>>> through, so how do I easily find out?
> > > > > > > > >>>>>> 
> > > > > > > > >>>>>>> But in this case it's vfio device itself that is sized so for sure you
> > > > > > > > >>>>>>> know it's MMIO.
> > > > > > > > >>>>>> 
> > > > > > > > >>>>>> How so?  I have a MemoryListener as described above and pass everything
> > > > > > > > >>>>>> through to the IOMMU.  I suppose I could look through all the
> > > > > > > > >>>>>> VFIODevices and check if the MemoryRegion matches, but that seems really
> > > > > > > > >>>>>> ugly.
> > > > > > > > >>>>>> 
> > > > > > > > >>>>>>> Maybe you will have same issue if there's another device with a 64 bit
> > > > > > > > >>>>>>> bar though, like ivshmem?
> > > > > > > > >>>>>> 
> > > > > > > > >>>>>> Perhaps, I suspect I'll see anything that registers their BAR
> > > > > > > > >>>>>> MemoryRegion from memory_region_init_ram or memory_region_init_ram_ptr.
> > > > > > > > >>>>> 
> > > > > > > > >>>>> Must be a 64 bit BAR to trigger the issue though.
> > > > > > > > >>>>> 
> > > > > > > > >>>>>>>>> Second, it enables peer-to-peer DMA between devices, which is something
> > > > > > > > >>>>>>>>> that we might be able to take advantage of with GPU passthrough.
> > > > > > > > >>>>>>>>> 
> > > > > > > > >>>>>>>>>>> Prior to this change, there was no re-map with the fffffffffebe0000
> > > > > > > > >>>>>>>>>>> address, presumably because it was beyond the address space of the PCI
> > > > > > > > >>>>>>>>>>> window.  This address is clearly not in a PCI MMIO space, so why are we
> > > > > > > > >>>>>>>>>>> allowing it to be realized in the system address space at this location?
> > > > > > > > >>>>>>>>>>> Thanks,
> > > > > > > > >>>>>>>>>>> 
> > > > > > > > >>>>>>>>>>> Alex
> > > > > > > > >>>>>>>>>> 
> > > > > > > > >>>>>>>>>> Why do you think it is not in PCI MMIO space?
> > > > > > > > >>>>>>>>>> True, CPU can't access this address but other pci devices can.
> > > > > > > > >>>>>>>>> 
> > > > > > > > >>>>>>>>> What happens on real hardware when an address like this is programmed to
> > > > > > > > >>>>>>>>> a device?  The CPU doesn't have the physical bits to access it.  I have
> > > > > > > > >>>>>>>>> serious doubts that another PCI device would be able to access it
> > > > > > > > >>>>>>>>> either.  Maybe in some limited scenario where the devices are on the
> > > > > > > > >>>>>>>>> same conventional PCI bus.  In the typical case, PCI addresses are
> > > > > > > > >>>>>>>>> always limited by some kind of aperture, whether that's explicit in
> > > > > > > > >>>>>>>>> bridge windows or implicit in hardware design (and perhaps made explicit
> > > > > > > > >>>>>>>>> in ACPI).  Even if I wanted to filter these out as noise in vfio, how
> > > > > > > > >>>>>>>>> would I do it in a way that still allows real 64bit MMIO to be
> > > > > > > > >>>>>>>>> programmed.  PCI has this knowledge, I hope.  VFIO doesn't.  Thanks,
> > > > > > > > >>>>>>>>> 
> > > > > > > > >>>>>>>>> Alex
> > > > > > > > >>>>>>> 
> > > > > > > > >>>>>>> AFAIK PCI doesn't have that knowledge as such. PCI spec is explicit that
> > > > > > > > >>>>>>> full 64 bit addresses must be allowed and hardware validation
> > > > > > > > >>>>>>> test suites normally check that it actually does work
> > > > > > > > >>>>>>> if it happens.
> > > > > > > > >>>>>> 
> > > > > > > > >>>>>> Sure, PCI devices themselves, but the chipset typically has defined
> > > > > > > > >>>>>> routing, that's more what I'm referring to.  There are generally only
> > > > > > > > >>>>>> fixed address windows for RAM vs MMIO.
> > > > > > > > >>>>> 
> > > > > > > > >>>>> The physical chipset? Likely - in the presence of IOMMU.
> > > > > > > > >>>>> Without that, devices can talk to each other without going
> > > > > > > > >>>>> through chipset, and bridge spec is very explicit that
> > > > > > > > >>>>> full 64 bit addressing must be supported.
> > > > > > > > >>>>> 
> > > > > > > > >>>>> So as long as we don't emulate an IOMMU,
> > > > > > > > >>>>> guest will normally think it's okay to use any address.
> > > > > > > > >>>>> 
> > > > > > > > >>>>>>> Yes, if there's a bridge somewhere on the path that bridge's
> > > > > > > > >>>>>>> windows would protect you, but pci already does this filtering:
> > > > > > > > >>>>>>> if you see this address in the memory map this means
> > > > > > > > >>>>>>> your virtual device is on root bus.
> > > > > > > > >>>>>>> 
> > > > > > > > >>>>>>> So I think it's the other way around: if VFIO requires specific
> > > > > > > > >>>>>>> address ranges to be assigned to devices, it should give this
> > > > > > > > >>>>>>> info to qemu and qemu can give this to guest.
> > > > > > > > >>>>>>> Then anything outside that range can be ignored by VFIO.
> > > > > > > > >>>>>> 
> > > > > > > > >>>>>> Then we get into deficiencies in the IOMMU API and maybe VFIO.  There's
> > > > > > > > >>>>>> currently no way to find out the address width of the IOMMU.  We've been
> > > > > > > > >>>>>> getting by because it's safely close enough to the CPU address width to
> > > > > > > > >>>>>> not be a concern until we start exposing things at the top of the 64bit
> > > > > > > > >>>>>> address space.  Maybe I can safely ignore anything above
> > > > > > > > >>>>>> TARGET_PHYS_ADDR_SPACE_BITS for now.  Thanks,
> > > > > > > > >>>>>> 
> > > > > > > > >>>>>> Alex
> > > > > > > > >>>>> 
> > > > > > > > >>>>> I think it's not related to target CPU at all - it's a host limitation.
> > > > > > > > >>>>> So just make up your own constant, maybe depending on host architecture.
> > > > > > > > >>>>> Long term add an ioctl to query it.
> > > > > > > > >>>> 
> > > > > > > > >>>> It's a hardware limitation which I'd imagine has some loose ties to the
> > > > > > > > >>>> physical address bits of the CPU.
> > > > > > > > >>>> 
> > > > > > > > >>>>> Also, we can add a fwcfg interface to tell bios that it should avoid
> > > > > > > > >>>>> placing BARs above some address.
> > > > > > > > >>>> 
> > > > > > > > >>>> That doesn't help this case, it's a spurious mapping caused by sizing
> > > > > > > > >>>> the BARs with them enabled.  We may still want such a thing to feed into
> > > > > > > > >>>> building ACPI tables though.
> > > > > > > > >>> 
> > > > > > > > >>> Well the point is that if you want BIOS to avoid
> > > > > > > > >>> specific addresses, you need to tell it what to avoid.
> > > > > > > > >>> But neither BIOS nor ACPI actually cover the range above
> > > > > > > > >>> 2^48 ATM so it's not a high priority.
> > > > > > > > >>> 
> > > > > > > > >>>>> Since it's a vfio limitation I think it should be a vfio API, along the
> > > > > > > > >>>>> lines of vfio_get_addr_space_bits(void).
> > > > > > > > >>>>> (Is this true btw? legacy assignment doesn't have this problem?)
> > > > > > > > >>>> 
> > > > > > > > >>>> It's an IOMMU hardware limitation, legacy assignment has the same
> > > > > > > > >>>> problem.  It looks like legacy will abort() in QEMU for the failed
> > > > > > > > >>>> mapping and I'm planning to tighten vfio to also kill the VM for failed
> > > > > > > > >>>> mappings.  In the short term, I think I'll ignore any mappings above
> > > > > > > > >>>> TARGET_PHYS_ADDR_SPACE_BITS,
> > > > > > > > >>> 
> > > > > > > > >>> That seems very wrong. It will still fail on an x86 host if we are
> > > > > > > > >>> emulating a CPU with full 64 bit addressing. The limitation is on the
> > > > > > > > >>> host side there's no real reason to tie it to the target.
> > > > > > > > > 
> > > > > > > > > I doubt vfio would be the only thing broken in that case.
> > > > > > > > > 
> > > > > > > > >>>> long term vfio already has an IOMMU info
> > > > > > > > >>>> ioctl that we could use to return this information, but we'll need to
> > > > > > > > >>>> figure out how to get it out of the IOMMU driver first.
> > > > > > > > >>>> Thanks,
> > > > > > > > >>>> 
> > > > > > > > >>>> Alex
> > > > > > > > >>> 
> > > > > > > > >>> Short term, just assume 48 bits on x86.
> > > > > > > > > 
> > > > > > > > > I hate to pick an arbitrary value since we have a very specific mapping
> > > > > > > > > we're trying to avoid.  Perhaps a better option is to skip anything
> > > > > > > > > where:
> > > > > > > > > 
> > > > > > > > >        MemoryRegionSection.offset_within_address_space >
> > > > > > > > >        ~MemoryRegionSection.offset_within_address_space
> > > > > > > > > 
> > > > > > > > >>> We need to figure out what's the limitation on ppc and arm -
> > > > > > > > >>> maybe there's none and it can address full 64 bit range.
> > > > > > > > >> 
> > > > > > > > >> IIUC on PPC and ARM you always have BAR windows where things can get mapped into. Unlike x86 where the full phyiscal address range can be overlayed by BARs.
> > > > > > > > >> 
> > > > > > > > >> Or did I misunderstand the question?
> > > > > > > > > 
> > > > > > > > > Sounds right, if either BAR mappings outside the window will not be
> > > > > > > > > realized in the memory space or the IOMMU has a full 64bit address
> > > > > > > > > space, there's no problem.  Here we have an intermediate step in the BAR
> > > > > > > > > sizing producing a stray mapping that the IOMMU hardware can't handle.
> > > > > > > > > Even if we could handle it, it's not clear that we want to.  On AMD-Vi
> > > > > > > > > the IOMMU pages tables can grow to 6-levels deep.  A stray mapping like
> > > > > > > > > this then causes space and time overhead until the tables are pruned
> > > > > > > > > back down.  Thanks,
> > > > > > > > 
> > > > > > > > I thought sizing is hard defined as a set to
> > > > > > > > -1? Can't we check for that one special case and treat it as "not mapped, but tell the guest the size in config space"?
> > > > > > > 
> > > > > > > PCI doesn't want to handle this as anything special to differentiate a
> > > > > > > sizing mask from a valid BAR address.  I agree though, I'd prefer to
> > > > > > > never see a spurious address like this in my MemoryListener.
> > > > > > 
> > > > > > It's more a can't than doesn't want to: it's a 64 bit BAR, it's not
> > > > > > set to all ones atomically.
> > > > > > 
> > > > > > Also, while it doesn't address this fully (same issue can happen
> > > > > > e.g. with ivshmem), do you think we should distinguish these BARs mapped
> > > > > > from vfio / device assignment in qemu somehow?
> > > > > > 
> > > > > > In particular, even when it has sane addresses:
> > > > > > device really can not DMA into its own BAR, that's a spec violation
> > > > > > so in theory can do anything including crashing the system.
> > > > > > I don't know what happens in practice but
> > > > > > if you are programming IOMMU to forward transactions back to
> > > > > > device that originated it, you are not doing it any favors.
> > > > > 
> > > > > I might concede that peer-to-peer is more trouble than it's worth if I
> > > > > had a convenient way to ignore MMIO mappings in my MemoryListener, but I
> > > > > don't.
> > > > 
> > > > Well for VFIO devices you are creating these mappings so we surely
> > > > can find a way for you to check that.
> > > > Doesn't each segment point back at the memory region that created it?
> > > > Then you can just check that.
> > > 
> > > It's a fairly heavy-weight search and it only avoid vfio devices, so it
> > > feels like it's just delaying a real solution.
> > 
> > Well there are several problems.
> > 
> > That device get its own BAR programmed
> > as a valid target in IOMMU is in my opinion a separate bug,
> > and for *that* it's a real solution.
> 
> Except the side-effect of that solution is that it also disables
> peer-to-peer since we do not use separate IOMMU domains per device.  In
> fact, we can't guarantee that it's possible to use separate IOMMU
> domains per device.

Interesting. I guess we can make it work if there's a single
device, this will cover many users though not all of them.

>  So, the cure is worse than the disease.

Worth checking what's worse. Want to try making device DMA
into its own BAR and see what crashes? It's a spec violation
so all bets are off but we can try to see at least some systems.

> > > > >  Self-DMA is really not the intent of doing the mapping, but
> > > > > peer-to-peer does have merit.
> > > > > 
> > > > > > I also note that if someone tries zero copy transmit out of such an
> > > > > > address, get user pages will fail.
> > > > > > I think this means tun zero copy transmit needs to fall-back
> > > > > > on copy from user on get user pages failure.
> > > > > > 
> > > > > > Jason, what's tour thinking on this?
> > > > > > 
> > > > > 
> > > > > 
> > > 
> > > 
> 
>