[Xen-devel] Ping: Re: [PATCH 2/3] VT-d: correct dma_msi_set_affinity()

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 09.12.16 at 09:47, <JBeulich@xxxxxxxx> wrote:
>>>> On 08.12.16 at 18:33, <andrew.cooper3@xxxxxxxxxx> wrote:
>> On 08/12/16 16:01, Jan Beulich wrote:
>>> That commit ("VT-d: use msi_compose_msg()) together with 15aa6c6748
>> 
>> Which commit?
> 
> Oops - initially I had intended the title to include the hash: 83cd2038fe.
> I've adjusted the text.
> 
>>> ("amd iommu: use base platform MSI implementation") introducing the use
>>> of a per-CPU scratch CPU mask went too far: dma_msi_set_affinity() may,
>>> at least in theory, be called in interrupt context, and hence the use
>>> of that scratch variable is not correct.
>>>
>>> Since the function overwrites the destination information anyway,
>>> allow msi_compose_msg() to be called with a NULL CPU mask, avoiding the
>>> use of that scratch variable.
>> 
>> Which function overwrites what?  I can't see dma_msi_set_affinity()
>> doing anything to clobber msg.dest32, so I don't understand why this
>> change is correct.
> 
> msg.dest32 simply isn't being used. msg is local to that function, so
> all that matters is which fields the function consumes. Is uses only
> address and data, and updates address to properly specify the
> intended destination. To guard against stale data (in
> iommu->msi.msg), it may be reasonable to nevertheless set dest32
> before storing msg into that field.

Any further thoughts here? Do I need to resend with that one line
added?

Jan


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