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

[Alex Williamson]

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 <address@hidden>
> > > 
> > > 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 <address@hidden>
> > > Signed-off-by: Paolo Bonzini <address@hidden>
> > > Signed-off-by: Michael S. Tsirkin <address@hidden>
> > > ---
> > >  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.

> >  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.
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