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

[Avi K. <av...@qu...>]

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.

-- 
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