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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> 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.

Jan


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