• Dean Nelson's avatar
    e1000: don't enable dma receives until after dma address has been setup · d5bc77a2
    Dean Nelson authored
    Doing an 'ifconfig ethN down' followed by an 'ifconfig ethN up' on a qemu-kvm
    guest system configured with two e1000 NICs can result in an 'unable to handle
    kernel paging request at 0000000100000000' or 'bad page map in process ...' or
    something similar.
    
    These result from a 4096-byte page being corrupted with the following two-word
    pattern (16-bytes) repeated throughout the entire page:
    
      0x0000000000000000
      0x0000000100000000
    
    There can be other bits set as well. What is a constant is that the 2nd word
    has the 32nd bit set. So one could see:
    
            :
      0x0000000000000000
      0x0000000100000000
      0x0000000000000000
      0x0000000172adc067    <<< bad pte
      0x800000006ec60067
      0x0000000700000040
      0x0000000000000000
      0x0000000100000000
            :
    
    Which came from from a process' page table I dumped out when the marked line
    was seen as bad by print_bad_pte().
    
    The repeating pattern represents the e1000's two-word receive descriptor:
    
    struct e1000_rx_desc {
            __le64 buffer_addr;   /* Address of the descriptor's data buffer */
            __le16 length;        /* Length of data DMAed into data buffer */
            __le16 csum;          /* Packet checksum */
            u8 status;            /* Descriptor status */
            u8 errors;            /* Descriptor Errors */
            __le16 special;
    };
    
    And the 32nd bit of the 2nd word maps to the 'u8 status' member, and
    corresponds to E1000_RXD_STAT_DD which indicates the descriptor is done.
    
    The corruption appears to result from the following...
    
     . An 'ifconfig ethN down' gets us into e1000_close(), which through a number
       of subfunctions results in:
         1. E1000_RCTL_EN being cleared in RCTL register.  [e1000_down()]
         2. dma_free_coherent() being called.  [e1000_free_rx_resources()]
    
     . An 'ifconfig ethN up' gets us into e1000_open(), which through a number of
       subfunctions results in:
         1. dma_alloc_coherent() being called.  [e1000_setup_rx_resources()]
         2. E1000_RCTL_EN being set in RCTL register.  [e1000_setup_rctl()]
         3. E1000_RCTL_EN being cleared in RCTL register.  [e1000_configure_rx()]
         4. RDLEN, RDBAH and RDBAL registers being set to reflect the dma page
            allocated in step 1.  [e1000_configure_rx()]
         5. E1000_RCTL_EN being set in RCTL register.  [e1000_configure_rx()]
    
    During the 'ifconfig ethN up' there is a window opened, starting in step 2
    where the receives are enabled up until they are disabled in step 3, in which
    the address of the receive descriptor dma page known by the NIC is still the
    previous one which was freed during the 'ifconfig ethN down'. If this memory
    has been reallocated for some other use and the NIC feels so inclined, it will
    write to that former dma page with predictably unpleasant results.
    
    I realize that in the guest, we're dealing with an e1000 NIC that is software
    emulated by qemu-kvm. The problem doesn't appear to occur on bare-metal. Andy
    suspects that this is because in the emulator link-up is essentially instant
    and traffic can start flowing immediately. Whereas on bare-metal, link-up
    usually seems to take at least a few milliseconds. And this might be enough
    to prevent traffic from flowing into the device inside the window where
    E1000_RCTL_EN is set.
    
    So perhaps a modification needs to be made to the qemu-kvm e1000 NIC emulator
    to delay the link-up. But in defense of the emulator, it seems like a bad idea
    to enable dma operations before the address of the memory to be involved has
    been made known.
    
    The following patch no longer enables receives in e1000_setup_rctl() but leaves
    them however they were. It only enables receives in e1000_configure_rx(), and
    only after the dma address has been made known to the hardware.
    
    There are two places where e1000_setup_rctl() gets called. The one in
    e1000_configure() is followed immediately by a call to e1000_configure_rx(), so
    there's really no change functionally (except for the removal of the problem
    window. The other is in __e1000_shutdown() and is not followed by a call to
    e1000_configure_rx(), so there is a change functionally. But consider...
    
     . An 'ifconfig ethN down' (just as described above).
    
     . A 'suspend' of the system, which (I'm assuming) will find its way into
       e1000_suspend() which calls __e1000_shutdown() resulting in:
         1. E1000_RCTL_EN being set in RCTL register.  [e1000_setup_rctl()]
    
    And again we've re-opened the problem window for some unknown amount of time.
    Signed-off-by: default avatarAndy Gospodarek <andy@greyhouse.net>
    Signed-off-by: default avatarDean Nelson <dnelson@redhat.com>
    Signed-off-by: default avatarJeff Kirsher <jeffrey.t.kirsher@intel.com>
    d5bc77a2
e1000_main.c 136 KB