Re: [Xen-devel] [PATCH 1/2 v4] iommu/amd: Fix logic for clearing the IOMMU interrupt bits

[George Dunlap <George.Dunlap@xxxxxxxxxxxxx>]

On Mon, Jun 10, 2013 at 1:41 PM, Jan Beulich <JBeulich@xxxxxxxx> wrote:
> The IOMMU interrupt bits in the IOMMU status registers are
> "read-only, and write-1-to-clear (RW1C).  Therefore, the existing
> logic which reads the register, set the bit, and then writing back
> the values could accidentally clear certain bits if it has been set.
>
> The correct logic would just be writing only the value which only
> set the interrupt bits, and leave the rest to zeros.
>
> This patch also, clean up #define masks as Jan has suggested.
>
> Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@xxxxxxx>
>
> With iommu_interrupt_handler() properly having got switched its readl()
> from status to control register, the subsequent writel() needed to be
> switched too (and the RW1C comment there was bogus).
>
> Some of the cleanup went too far - undone.
>
> Further, with iommu_interrupt_handler() now actually disabling the
> interrupt sources, they also need to get re-enabled by the tasklet once
> it finished processing the respective log.
>
> Finally, guest write emulation to the status register needs to be done
> with the RW1C (and RO for all other bits) semantics in mind too.
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

What's the impact of this as a bug?

This looks like it has a fairly high risk of introducing regressions
in existing working system.  So unless it has a pretty wide impact, I
think we should wait and include this in 4.3.1.

 -George

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