Re: [Xen-devel] [PATCh][VT] partial PIC to HV patch

[Keir Fraser <Keir.Fraser@xxxxxxxxxxxx>]

On 22 Sep 2005, at 10:33, Dong, Eddie wrote:

> Keir:
>         This is patch to move PIC mask/EOI access to HV for performance
> and we saw about 5% performance increasement with this patch.
I'm not sure about this. I don't much like splitting the implementation 
of a device across both Xen and qemu -- I'd rather implement entirely 
within one or the other. Our eventual aim should be to move all non-Xen 
device models into the context of the vmx domain (running as a paravirt 
'mini domain' in rings 1 to 3 of the root vmcs). This might reduce 
latency enough for most devices to avoid distasteful 'split' 
device-model implementations.
In the case of local APICs, I/O APICs and the legacy PIC I can accept 
that, even in the above model there will probably be a measurable 
performance improvement to implementing these devices within Xen. But I 
think this should be done by moving their complete implementation into 
Xen and defining a clean interface to device models running in domain0 
and/or rings 1-3. In the case of the PIC, this could be done by having 
a virtualised 'interrupt wire' for each device (e.g., represent an 
'interrupt edge' by a special call into Xen that runs the PIC device 
model).
The other argument for putting the PIC and IO-APIC in Xen is that the 
PIT is already there, and having it call out into a non-Xen device 
model to send interrupts is rather gross.
 -- Keir


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