Re: [Xen-devel] [PATCH v2] AMD/intremap: Prevent use of per-device vector maps until irq logic is fixed

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 31.05.13 at 22:04, Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
> In an effort to get AMD systems back to a non-regressed state, introduce a 
> new
> type of vector map called per-device-global.  This uses per-device vector maps
> in the IOMMU, but uses a single used_vector map for the core IRQ logic.

So what's the reason for not simply using OPT_IRQ_VECTOR_MAP_GLOBAL
here?

> This patch is intended to be removed as soon as the per-device logic is fixed
> correctly.

As a last resort thing this may be acceptable, but I'd much favor to
fix this properly rather than hacking it like this. Hence I'd really like
to put up for discussion to instead use the patch[1] already posted
as preparatory for the multi-vector MSI support doing away with the
use of the vector for indexing the IRTE (and, in a second patch[2],
the enforcement of OPT_IRQ_VECTOR_MAP_PERDEV).

Also, overriding a command line request in the way you do is a
no-go imo - even if this would cause [theoretical] problems, we
ought to honor the request as long as we can't tell for sure that
this is going to break the specific system. That's even more so
since requesting per-device vector maps to be used on VT-d ought
to yield exactly the same effect, yet you don't override the mode
there.

Furthermore, if only MSI-X devices currently suffer from this, the
scalability effect this has (allowing nor more than about 200
vectors to be in use even on huge systems) would call for limiting
the effect to MSI-X capable devices (or perhaps even to devices
actually using MSI-X).

Jan

[1] http://lists.xen.org/archives/html/xen-devel/2013-04/msg01886.html
[2] http://lists.xen.org/archives/html/xen-devel/2013-04/msg01887.html


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