Re: [Qemu-devel] [PATCH 0/5][RFC] virtio-net: MAC filtering

[Avi Kivity <avi@redhat.com>]

Anthony Liguori wrote:
> Paul Brook wrote:
>>> A concern here is the growing size of the virtio-net I/O port space
>>> config.  This series brings it up to 256 bytes with PCI resource
>>> rounding.  The VLAN filter bitmap would increase that by another 512
>>> bytes, making it 1kB and limiting us to something less than 64 such
>>> devices per guest.  Is anyone worried?  Should filter tables live in
>>> MMIO space for virtio devices?  I'll send out the guest side patches 
>>> for
>>> virtio-net in a separate thread.  Thanks,
>>>     
>>
>> This is one reason why IO ports are a reallybad idea. Use memory 
>> mapped register spaces like any other sane system and you won't have 
>> a problem.
>>   
>
> IO ports are much faster for notification than MMIO in KVM which is 
> why the space is currently IO ports.  It was never meant to hold very 
> large amounts of data.

We can, btw, fix the mmio speed issue by adding two new hypercalls: 
mmio_read() and mmio_write().  We could then hook <asm/io.h> to use the 
hypercalls instead of reading directly.  This would speed up most 
emulated devices, not just virtio.

I don't know whether Windows drivers access mmio using helpers or directly.

-- 
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