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

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

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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