linux-kernel - Re: [PATCH v3 01/11] PCI/P2PDMA: Support peer-to-peer memory

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Date:   Tue, 13 Mar 2018 15:53:18 -0400
From:   Sinan Kaya <okaya@...eaurora.org>
To:     Logan Gunthorpe <logang@...tatee.com>,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-nvme@...ts.infradead.org, linux-rdma@...r.kernel.org,
        linux-nvdimm@...ts.01.org, linux-block@...r.kernel.org
Cc:     Stephen Bates <sbates@...thlin.com>,
        Christoph Hellwig <hch@....de>, Jens Axboe <axboe@...nel.dk>,
        Keith Busch <keith.busch@...el.com>,
        Sagi Grimberg <sagi@...mberg.me>,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Jason Gunthorpe <jgg@...lanox.com>,
        Max Gurtovoy <maxg@...lanox.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Jérôme Glisse <jglisse@...hat.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Alex Williamson <alex.williamson@...hat.com>
Subject: Re: [PATCH v3 01/11] PCI/P2PDMA: Support peer-to-peer memory

On 3/13/2018 3:19 PM, Logan Gunthorpe wrote:
> 
> 
> On 13/03/18 01:10 PM, Sinan Kaya wrote:
>> I was thinking of this for the pci_p2pdma_add_client() case for the
>> parent pointer.
>>
>> +struct pci_p2pdma_client {
>> +	struct list_head list;
>> +	struct pci_dev *client;
>> +	struct pci_dev *provider;
>> +};
> 
> Yeah, that structure only exists in a list owned by the client and we
> only check the upstream bridge once per entry so I don't see the point.
> 
>> But then, Why bother searching for the switch at all?
> 
> Huh? We have to make sure all the client and provider devices are behind
> the same switch. How can we do that without "searching" for the switch?
> 

Sorry, I was thinking of ACS case you described below. The only thing code
cares is if the device is behind a switch or not at this moment.

> In the ACS case, we only disable ACS on downstream ports of switches. No
> sense disabling it globally as that's worse from an isolation point of
> view and not worth it given we require all P2P transactions to be behind
> a switch.

I agree disabling globally would be bad. Somebody can always say I have
ten switches on my system. I want to do peer-to-peer on one switch only. Now,
this change weakened security for the other switches that I had no intention
with doing P2P.

Isn't this a problem?

Can we specify the BDF of the downstream device we want P2P with during boot via
kernel command line?

> 
>> Even if the switch is there, there is no guarantee that it is currently
>> being used for P2P.
> 
> IOMMU groups are set at boot time and, at present, there's no way to
> dynamically change ACS bits without messing up the groups. So switches
> not used for P2P will not have ACS enabled when CONFIG_PCI_P2PDMA is set
> and I don't know of any good solution to that. Please see the ACS
> discussions in v1 and v2.

Given the implementation limitations, this might be OK as a short-term
solution. 

It depends on if Alex is comfortable with this.

> 
>> It seems that we are going with the assumption that enabling this config
>> option implies you want P2P, then we can simplify this code as well.
> 
> How so?
> 
> Logan
> 
> 
> 


-- 
Sinan Kaya
Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc.
Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.