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[Xen-devel] Re: [PATCH RFC] Per-CPU vector for Xen.

To: "Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx>
Subject: [Xen-devel] Re: [PATCH RFC] Per-CPU vector for Xen.
From: Keir Fraser <keir.fraser@xxxxxxxxxxxxx>
Date: Tue, 18 Aug 2009 07:42:27 +0100
Cc: "xen-devel@xxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxx>, "Kay, Allen M" <allen.m.kay@xxxxxxxxx>, "Jiang, Yunhong" <yunhong.jiang@xxxxxxxxx>, "Dong, Eddie" <eddie.dong@xxxxxxxxx>, "Yang, Xiaowei" <xiaowei.yang@xxxxxxxxx>, "Li, Xin" <xin.li@xxxxxxxxx>
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On 18/08/2009 06:45, "Zhang, Xiantao" <xiantao.zhang@xxxxxxxxx> wrote:

> Hi, Keir
>    Another issue or limitation is also found when we tested Per-CPU vector
> patch at our side. Currenlty, Xen uses fixmap to access MSI-X resouce, (e.g.
> msi-x tables), 32 pages  512 pages are reserved for 32-pae and x86_64
> separately in fixmap section, but  each MSI-X capable device(regardless of
> real device or virtual function) at least needs one page to map its resource,
> so these pages may easily run out with these devices, espeically on 32-bit
> platforms. For 64-bit platforms, we can reserve more pages to fix the issue,
> but for 32-bit platforms, it is not always safe to increase the number of
> pages due to limited virtual address space.  We have one optional solution to
> fix the issue through dynamically map/unmap MSI-X tables when access it, but
> the concern is that it may cost much due to frequent access for the resource.
> What's suggestion or good idea to address the issue ?  Thanks!
> Xiantao

A lower-performance option for 32-bit Xen would be fine by me. Very few
people should have reason not to run 64-bit Xen now. Those who do may well
have old systems with no/few MSI-x devices.

 -- Keir

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