Re: [Xen-devel] [PATCH for-4.10 v2] passthrough/vtd: Don't DMA to the stack in queue_invalidate_wait()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 20/10/17 08:12, Jan Beulich wrote:
>>>> On 19.10.17 at 18:22, <andrew.cooper3@xxxxxxxxxx> wrote:
>> DMA-ing to the stack is generally considered bad practice.  In this case, if 
>> a
>> timeout occurs because of a sluggish device which is processing the request,
>> the completion notification will corrupt the stack of a subsequent deeper 
>> call
>> tree.
>>
>> Place the poll_slot in a percpu area and DMA to that instead.
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
> Please could you extend the commit message to state the issue
> remaining with using a single per-CPU slot? With that
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>

How about this?

Note: This change does not address other issues with the current
implementation, such as once a timeout has been suffered, subsequent
completions can't be correlated with their requests.

> albeit ...
>
>> @@ -167,7 +169,7 @@ static int __must_check queue_invalidate_wait(struct 
>> iommu *iommu,
>>      qinval_entry->q.inv_wait_dsc.lo.res_1 = 0;
>>      qinval_entry->q.inv_wait_dsc.lo.sdata = QINVAL_STAT_DONE;
>>      qinval_entry->q.inv_wait_dsc.hi.res_1 = 0;
>> -    qinval_entry->q.inv_wait_dsc.hi.saddr = virt_to_maddr(&poll_slot) >> 2;
>> +    qinval_entry->q.inv_wait_dsc.hi.saddr = virt_to_maddr(this_poll_slot) 
>> >> 2;
> ... this one is still a literal number rather than something allowing
> to associate back where that value is coming from (but since you're
> not introducing it here, I also won't insist on you changing it in this
> patch).

I don't understand.  What is still a literal number?

The sdata field is software-chosen, so one option would be to count the
number of wait descriptors we've requested of the hardware, and have it
echo that back.  That would allow us to detect if it is out of date.

~Andrew

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