Re: [Qemu-devel] [PATCH 2/2] e1000: unconditionally enable bus mastering

[Stefan Hajnoczi <stefanha@gmail.com>]

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On Thu, Dec 18, 2014 at 05:22:19PM +0800, Amos Kong wrote:
> After enabled network debug of e1000 in Win2012-64r2 guest,
> Bus mastering of e1000 can't be enabled by e1000 driver. It
> caused guest can't get IP address.
> 
>   # bcdedit /debug on
>   # bcdedit /dbgsettings net hostip:192.168.122.100 port:50000
>   (We can use non-existed IP here, it's just used to pass the
>    setup, not really use it)
> 
> If we disable debug function, e1000 driver can enable bus
> mastering bit successfully, guest network is fine.
> 
> This patch changed e1000 backend to enalbe bus mastering
> unconditionally as a workaround.
> 
> Signed-off-by: Amos Kong <akong@redhat.com>
> ---
>  hw/net/e1000.c | 11 +++++++++--
>  1 file changed, 9 insertions(+), 2 deletions(-)
> 
> diff --git a/hw/net/e1000.c b/hw/net/e1000.c
> index ec9224b..82829ae 100644
> --- a/hw/net/e1000.c
> +++ b/hw/net/e1000.c
> @@ -1544,8 +1544,15 @@ static void e1000_write_config(PCIDevice *pci_dev, uint32_t address,
>  
>      pci_default_write_config(pci_dev, address, val, len);
>  
> -    if (range_covers_byte(address, len, PCI_COMMAND) &&
> -        (pci_dev->config[PCI_COMMAND] & PCI_COMMAND_MASTER)) {
> +    if (range_covers_byte(address, len, PCI_COMMAND)) {
> +        /*
> +         * Some guest (eg: Win2012-64r2) doesn't enable bus mastering
> +         * correctly, it caused guest network down. So we unconditionally
> +         * enable PCI bus mastering and BM memory region for e1000 as
> +         * a workaround.
> +         */
> +        pci_dev->config[PCI_COMMAND] |= PCI_COMMAND_MASTER;
> +        memory_region_set_enabled(&pci_dev->bus_master_enable_region, true);
>          qemu_flush_queued_packets(qemu_get_queue(s->nic));
>          start_xmit(s);
>      }

This is weird.

Are you sure there's not some guest behavior missing like the NIC option
ROM leaving bus mastering enabled after the BIOS/EFI has booted, causing
Windows debug to work on physical machines?

Before we merge a hack like this we should understand the problem 100%.

Stefan

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