linux-kernel - Re: [PATCH v2 04/10] PCI/P2PDMA: Clear ACS P2P flags for all devices behind switches

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Date:   Mon, 5 Mar 2018 16:28:21 -0600
From:   Bjorn Helgaas <helgaas@...nel.org>
To:     Logan Gunthorpe <logang@...tatee.com>
Cc:     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,
        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 v2 04/10] PCI/P2PDMA: Clear ACS P2P flags for all devices
 behind switches

On Thu, Mar 01, 2018 at 12:13:10PM -0700, Logan Gunthorpe wrote:
> 
> 
> On 01/03/18 11:02 AM, Bjorn Helgaas wrote:
> > >   void pci_enable_acs(struct pci_dev *dev)
> > >   {
> > > +	if (pci_p2pdma_disable_acs(dev))
> > > +		return;
> > 
> > This doesn't read naturally to me.  I do see that when
> > CONFIG_PCI_P2PDMA is not set, pci_p2pdma_disable_acs() does nothing
> > and returns 0, so we'll go ahead and try to enable ACS as before.
> > 
> > But I think it would be clearer to have an #ifdef CONFIG_PCI_P2PDMA
> > right here so it's more obvious that we only disable ACS when it's
> > selected.
> 
> I could do this... however, I wrote it this way because I've read Linus
> dislikes using #ifdef's inside function bodies and I personally agree with
> that sentiment.

I try to avoid #ifdefs too, but in this case the plain reading of the
code makes no sense (we're in a function called "enable_acs()", and
the first thing we do is call a function to "disable_acs()".

Disabling ACS is scary for all the security reasons mentioned
elsewhere, so a reader who knows what ACS does and cares about
virtualization and security *has* to go look up the definition of
pci_p2pdma_disable_acs().

If you put the #ifdef right here, then it's easier to read because we
can see that "oh, this is a special and uncommon case that I can
probably ignore".

Bjorn