linux-kernel - Re: [PATCH 04/12] swiotlb-xen: ensure to issue well-formed XENMEM

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Message-ID: <b93284dc-782e-b455-96fb-1c890ee42560@suse.com>
Date:   Wed, 15 Sep 2021 10:21:51 +0200
From:   Jan Beulich <jbeulich@...e.com>
To:     Stefano Stabellini <sstabellini@...nel.org>
Cc:     Juergen Gross <jgross@...e.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        lkml <linux-kernel@...r.kernel.org>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>
Subject: Re: [PATCH 04/12] swiotlb-xen: ensure to issue well-formed
 XENMEM_exchange requests

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@...e.com>
>>>>>>> ---
>>>>>>> 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