Re: [PATCH 04/12] swiotlb-xen: ensure to issue well-formed XENMEM_exchange requests

[Jan Beulich <jbeulich@xxxxxxxx>]

On 15.09.2021 03:54, Stefano Stabellini wrote:
> On Tue, 14 Sep 2021, Stefano Stabellini wrote:
>> On Tue, 14 Sep 2021, Jan Beulich wrote:
>>> On 13.09.2021 22:31, Stefano Stabellini wrote:
>>>> On Mon, 13 Sep 2021, Jan Beulich wrote:
>>>>> On 11.09.2021 01:14, Stefano Stabellini wrote:
>>>>>> On Tue, 7 Sep 2021, Jan Beulich wrote:
>>>>>>> While the hypervisor hasn't been enforcing this, we would still better
>>>>>>> avoid issuing requests with GFNs not aligned to the requested order.
>>>>>>>
>>>>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>>>>> ---
>>>>>>> I wonder how useful it is to include the alignment in the panic()
>>>>>>> message.
>>>>>>
>>>>>> Not very useful given that it is static. I don't mind either way but you
>>>>>> can go ahead and remove it if you prefer (and it would make the line
>>>>>> shorter.)
>>>>>>
>>>>>>
>>>>>>> I further wonder how useful it is to wrap "bytes" in
>>>>>>> PAGE_ALIGN(), when it is a multiple of a segment's size anyway (or at
>>>>>>> least was supposed to be, prior to "swiotlb-xen: maintain slab count
>>>>>>> properly").
>>>>>>
>>>>>> This one I would keep, to make sure to print out the same amount passed
>>>>>> to memblock_alloc.
>>>>>
>>>>> Oh - if I was to drop it from the printk(), I would have been meaning to
>>>>> also drop it there. If it's useless, then it's useless everywhere.
>>>>
>>>> That's fine too
>>>
>>> Thanks, I'll see about dropping that then.
>>>
>>> Another Arm-related question has occurred to me: Do you actually
>>> mind the higher-than-necessary alignment there? If so, a per-arch
>>> definition of the needed alignment would need introducing. Maybe
>>> that could default to PAGE_SIZE, allowing Arm and alike to get away
>>> without explicitly specifying a value ...
>>
>> Certainly a patch like that could be good. Given that it is only one
>> allocation I was assuming that the higher-than-necessary alignment
>> wouldn't be a problem worth addressing (and I cannot completely rule out
>> that one day we might have to use XENMEM_exchange on ARM too).
> 
> Also this code is currently #ifdef CONFIG_X86

Oh, good point. When writing the patch I did take this into consideration,
but I had managed to forget that aspect in the meantime. No adjustment to
this effect needed then.

Jan