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Re: [Xen-devel] [VTD][patch 0/5] HVM device assignment using vt-d

To: Keir Fraser <keir@xxxxxxxxxxxxx>, "Tian, Kevin" <kevin.tian@xxxxxxxxx>, Guy Zana <guy@xxxxxxxxxxxx>, "Kay, Allen M" <allen.m.kay@xxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxx>
Subject: Re: [Xen-devel] [VTD][patch 0/5] HVM device assignment using vt-d
From: Keir Fraser <keir@xxxxxxxxxxxxx>
Date: Thu, 31 May 2007 16:52:25 +0100
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On 31/5/07 16:40, "Keir Fraser" <keir@xxxxxxxxxxxxx> wrote:

> It'd be interesting to know how these two approaches compare
> performance-wise. I suppose yours should win, really, due to fewer physical
> interrupts.

One thing is that the polarity-switching approach is a slightly better fit
with the HVM interrupt logic. Currently interrupt sources and VIOAPIC are
not tightly bound together; they only interact by one waggling the virtual
intx wires and the other sampling that wire periodically (or synchronously
on +ve edges). Your approach requires a 'back channel' from the VIOAPIC code
back to physical interrupt code to call ->end(). It's kind of ugly. On the
other hand I suspect the polarity-switching code adds more stuff to the
phsyical interrupt subsystem, and your approach can certainly be supported,
probably by adding a bit more state (maybe just a single bit) per virtual
intx wire. Really we need to look at and measure each implementation...

 -- Keir

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