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

[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,

Alex