Re: MSI to memory?

[Helge Hafting]

Roland Dreier wrote:

>    Helge> Won't that put a bad load on the bus?  Someone else might
>    Helge> need it: * Another cpu in a smp system * Any device doing
>    Helge> bus-master transfers, even in a UP system
>
> Actually with MSI, the PCI device writes directly to a host address.
> In the proposed usage in this mail thread, the address is in host
> memory, so there's no bus load to poll the memory.  Presumably the
> memory will be pulled into cache for the duration of the poll loop, so
> there's not even any memory bandwidth consumed.  (Of course this only
> works on an architecture where PCI DMA is cache coherent)
>  
I see.  Still, this should only be used when we expect a
short wait only, similiar to situations when we use a spinlock.
Otherwise, the cpu could be put to better use by scheduling.

Helge Hafting
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