Subject: Re: [PATCH v3 05/15] xprtrdma: Remove last ib_reg_phys_mr() call site
To: Steve Wise <swise@opengridcomputing.com>,
        "'Jason Gunthorpe'" <jgunthorpe@obsidianresearch.com>
References: <20150720185624.10997.51574.stgit@manet.1015granger.net>
 <20150720190311.10997.12636.stgit@manet.1015granger.net>
 <55AD5B48.3010906@talpey.com> <20150720210544.GA9655@obsidianresearch.com>
 <015101d0c331$69e31d10$3da95730$@opengridcomputing.com>
 <55AD7065.8040809@talpey.com>
 <015701d0c33d$36b53110$a41f9330$@opengridcomputing.com>
 <55AD8F05.6070409@talpey.com>
 <000301d0c3c2$3f892a50$be9b7ef0$@opengridcomputing.com>
Cc: "'Chuck Lever'" <chuck.lever@oracle.com>, linux-rdma@vger.kernel.org,
        linux-nfs@vger.kernel.org
From: Tom Talpey <tom@talpey.com>
Message-ID: <55AEAFCE.60207@talpey.com>
Date: Tue, 21 Jul 2015 13:47:10 -0700
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Sender: linux-nfs-owner@vger.kernel.org

On 7/21/2015 7:33 AM, Steve Wise wrote:
>
>
>> -----Original Message-----
>> From: linux-rdma-owner@vger.kernel.org [mailto:linux-rdma-owner@vger.kernel.org] On Behalf Of Tom Talpey
>> Sent: Monday, July 20, 2015 7:15 PM
>> To: Steve Wise; 'Jason Gunthorpe'
>> Cc: 'Chuck Lever'; linux-rdma@vger.kernel.org; linux-nfs@vger.kernel.org
>> Subject: Re: [PATCH v3 05/15] xprtrdma: Remove last ib_reg_phys_mr() call site
>>
>> On 7/20/2015 3:41 PM, Steve Wise wrote:
>>>
>>>
>>>> -----Original Message-----
>>>> From: Tom Talpey [mailto:tom@talpey.com]
>>>> Sent: Monday, July 20, 2015 5:04 PM
>>>> To: Steve Wise; 'Jason Gunthorpe'
>>>> Cc: 'Chuck Lever'; linux-rdma@vger.kernel.org; linux-nfs@vger.kernel.org
>>>> Subject: Re: [PATCH v3 05/15] xprtrdma: Remove last ib_reg_phys_mr() call site
>>>>
>>>> On 7/20/2015 2:16 PM, Steve Wise wrote:
>>>>>
>>>>>
>>>>>> -----Original Message-----
>>>>>> From: linux-nfs-owner@vger.kernel.org [mailto:linux-nfs-owner@vger.kernel.org] On Behalf Of Jason Gunthorpe
>>>>>> Sent: Monday, July 20, 2015 4:06 PM
>>>>>> To: Tom Talpey; Steve Wise
>>>>>> Cc: Chuck Lever; linux-rdma@vger.kernel.org; linux-nfs@vger.kernel.org
>>>>>> Subject: Re: [PATCH v3 05/15] xprtrdma: Remove last ib_reg_phys_mr() call site
>>>>>>
>>>>>> On Mon, Jul 20, 2015 at 01:34:16PM -0700, Tom Talpey wrote:
>>>>>>> On 7/20/2015 12:03 PM, Chuck Lever wrote:
>>>>>>>> All HCA providers have an ib_get_dma_mr() verb. Thus
>>>>>>>> rpcrdma_ia_open() will either grab the device's local_dma_key if one
>>>>>>>> is available, or it will call ib_get_dma_mr() which is a 100%
>>>>>>>> guaranteed fallback.
>>>>>>>
>>>>>>> I recall that in the past, some providers did not support mapping
>>>>>>> all of the machine's potential physical memory with a single dma_mr.
>>>>>>> If an rnic did/does not support 44-ish bits of length per region,
>>>>>>> for example.
>>>>>>
>>>>>> Looks like you are right, but the standard in kernel is to require
>>>>>> ib_get_dma_mr, if the HCA can't do that, then it cannot be used on a
>>>>>> big memory machine with kernel ULPs.
>>>>>>
>>>>>> Looking deeper, both amso1100 and cxgb3 seem limited to 32 bits of
>>>>>> physical memory, and silently break all kernel ULPs if they are used
>>>>>> on a modern machine with > 4G.
>>>>>>
>>>>>> Is that right Steve?
>>>>>>
>>>>>
>>>>> Yes.
>>>>>
>>>>>> Based on that, should we remove the cxgb3 driver as well? Or at least
>>>>>> can you fix it up to at least fail get_dma_mr if there is too much
>>>>>> ram?
>>>>>>
>>>>>
>>>>> I would like to keep cxgb3 around.  I can add code to fail if the memory is > 32b.  Do you know how I get the amount of
>>> available
>>>>> ram?
>>>>
>>>> A) are you sure it's an unsigned length, i.e. is it really 31 bits?
>>>>
>>>
>>> yes.
>>>
>>>> B) why bother to check? Are machines with <4GB interesting, and worth
>>>> supporting a special optimization?
>>>
>>> No, but cxgb3 is still interesting to user applications, and perhaps NFSRDMA using FRMRs.
>>
>> I'm obviously not making myself clear. I am suggesting that cxgb3 fail
>> the ib_get_dma_mr() verb, regardless of installed memory.
>>
>> I am not suggesting it fail to load, or fail other memreg requests. It
>> should work normally in all other respects.
>
> Even with its limitation, doesn't it have utility for someone using cxgb3 in an embedded 32b environment?

Sure, do you mean making it conditional on #if sizeof(physaddr) <= 32?
That would make sense I guess.