linux-kernel - Re: [PATCH v7 3/8] arm64: introduce is_device_dma

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Date:	Fri, 7 Nov 2014 17:35:41 +0000
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
CC:	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <Will.Deacon@....com>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
	"Ian.Campbell@...rix.com" <Ian.Campbell@...rix.com>,
	"david.vrabel@...rix.com" <david.vrabel@...rix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v7 3/8] arm64: introduce is_device_dma_coherent

On Fri, 7 Nov 2014, Stefano Stabellini wrote:
> On Fri, 7 Nov 2014, Catalin Marinas wrote:
> > What I would like to see is xen_dma_map_page() also using hyp calls for
> > cache maintenance when !pfn_valid(), for symmetry with unmap. You would
> > need another argument to xen_dma_map_page() though to pass the real
> > device address or mfn (and on the map side you could simply check for
> > page_to_pfn(page) != mfn). For such cases, Xen swiotlb already handles
> > bouncing so you don't need dom0 swiotlb involved as well.
> 
> I can see that it would be nice to have map_page and unmap_page be
> symmetrical. However actually doing the map_page flush with an hypercall
> would slow things down. Hypercalls are slower than function calls. It is
> faster to do the cache flushing in dom0 if possible. In the map_page
> case we have the struct page so we can easily do it by calling the
> native dma_ops function.
> 
> Maybe I could just add a comment to explain the reason for the asymmetry?

Ah, but the problem is that map_page could allocate a swiotlb buffer
(actually it does on arm64) that without a corresponding unmap_page
call, would end up being leaked, right?

Oh well.. two hypercalls it is then :-/
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