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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 13.12.16 at 06:23, <kevin.tian@xxxxxxxxx> wrote:
>>  From: Jan Beulich [mailto:JBeulich@xxxxxxxx]
>> Sent: Friday, December 09, 2016 4:47 PM
>> 
>> >>> 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.
> 
> So do you plan to send v2 or stick with current version?

Well, so far I haven't heard back from Andrew, and hence didn't
plan on sending a v2 yet. If that addition is going to be the only
adjustment, I'm also not sure sending a v2 is actually necessary.

Jan


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