Re: [patch 0/3] QEMU/KVM: add support for 128 PCI slots (v2)

[Anthony Liguori <anthony@codemonkey.ws>]

Avi Kivity wrote:
> Marcelo Tosatti wrote:
>   
>> Add three PCI bridges to support 128 slots.
>>
>> Changes since v1:
>> - Remove I/O address range "support" (so standard PCI I/O space is used).
>> - Verify that there's no special quirks for 82801 PCI bridge.
>> - Introduce separate flat IRQ mapping function for non-SPARC targets.
>>
>>   
>>     
>
> I've cooled off on the 128 slot stuff, mainly because most real hosts 
> don't have them.  An unusual configuration will likely lead to problems 
> as most guest OSes and workloads will not have been tested thoroughly 
> with them.
>
> - it requires a large number of interrupts, which are difficult to 
> provide, and which it is hard to ensure all OSes support.  MSI is 
> relatively new.
> - is only a few interrupts are available, then each interrupt requires 
> scanning a large number of queues
>
> If we are to do this, then we need better tests than "80 disks show up".
>
> The alternative approach of having the virtio block device control up to 
> 16 disks allows having those 80 disks with just 5 slots (and 5 
> interrupts).  This is similar to the way traditional SCSI controllers 
> behave, and so should not surprise the guest OS.
>   

If you have a single virtio-blk device that shows up as 8 functions, we 
could achieve the same thing.  We can cheat with the interrupt handlers 
to avoid cache line bouncing too.  Plus, we can use PCI hotplug so we 
don't have to reinvent a new hotplug mechanism.

I'm inclined to think that ring sharing isn't as useful as it seems as 
long as we don't have indirect scatter gather lists.

Regards,

Anthony Liguori



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