linux-kernel - Re: [PATCH v4 09/10] swiotlb-xen: support autotranslate guests

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Date:	Fri, 30 Aug 2013 09:23:12 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
Cc:	xen-devel@...ts.xensource.com, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, Ian.Campbell@...rix.com
Subject: Re: [PATCH v4 09/10] swiotlb-xen: support autotranslate guests

> > Did you run any performance tests to see if adding the extra
> > spinlock (as the native SWIOTLB already has its own lock) and handling
> > of the tree is affecting it?

.. bla bla..

> I haven't done any measurements but consider that the spin_lock is
> already only used to access the red-black tree that keeps track of
> dma_addr -> phys_addr mappings.
> So it's taken at setup time once, then every time we call
> xen_bus_to_phys and the guest is an autotraslate guest.
> If the guest is not autotraslate there are no additional locks.
> 
> That makes me realize that we don't need any spin_locks at all: there
> are no risks of concurrent accesses and modifications of the tree
> because there are no changes on the tree once it's setup at boot time.
> We can get rid of the spin_lock entirely as concurrent read accesses on
> the tree are obviously fine.

Nice :-) I think that (and the goto) were the only concerns I had.

Thanks!
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