linux-kernel - Re: [PATCH v7 6/8] xen/arm/arm64: merge xen/mm32.c into xen/mm.c

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Date:	Fri, 7 Nov 2014 14:22:05 +0000
From:	Catalin Marinas <catalin.marinas@....com>
To:	Stefano Stabellini <stefano.stabellini@...citrix.com>
Cc:	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>,
	"Ian.Campbell@...rix.com" <Ian.Campbell@...rix.com>,
	"konrad.wilk@...cle.com" <konrad.wilk@...cle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"david.vrabel@...rix.com" <david.vrabel@...rix.com>,
	"linux-arm-kernel@...ts.infradead.org" 
	<linux-arm-kernel@...ts.infradead.org>
Subject: Re: [PATCH v7 6/8] xen/arm/arm64: merge xen/mm32.c into xen/mm.c

On Mon, Oct 27, 2014 at 03:09:26PM +0000, Stefano Stabellini wrote:
> Merge xen/mm32.c into xen/mm.c.
> As a consequence the code gets compiled on arm64 too: introduce a few
> compat functions to actually be able to compile it.
> 
> Signed-off-by: Stefano Stabellini <stefano.stabellini@...citrix.com>
> Acked-by: Ian Campbell <ian.campbell@...rix.com>

Since I missed the commit introducing mm32.c (340720be32d4 xen/arm:
reimplement xen_dma_unmap_page & friends), I'll add a retrospective NAK ;).

The main reason is the asymmetry between dma map and unmap. With host
swiotlb somehow getting !dma_capable(dev), you even risk leaking dom0
swiotlb bounce buffers (on arm64).

> --- a/arch/arm/xen/mm.c
> +++ b/arch/arm/xen/mm.c
[...]
> +/* functions called by SWIOTLB */
> +
> +static void dma_cache_maint(dma_addr_t handle, unsigned long offset,
> +       size_t size, enum dma_data_direction dir,
> +       void (*op)(const void *, size_t, int))
> +{
> +       unsigned long pfn;
> +       size_t left = size;
> +
> +       pfn = (handle >> PAGE_SHIFT) + offset / PAGE_SIZE;
> +       offset %= PAGE_SIZE;
> +
> +       do {
> +               size_t len = left;
> +               void *vaddr;
> +
> +               if (!pfn_valid(pfn))

Is this the pfn or the mfn? As you said in the previous email, there is
no mfn_to_pfn() conversion, so that's actually in another address space
where dom0 pfn_valid() would not make sense.

Do you use this as a check for foreign pages? If yes, is the mfn for
such pages guaranteed to be different from any valid dom0 pfn?

> +               {
> +                       /* TODO: cache flush */

What does this TODO mean here? Which cases are not covered yet?

> +               } else {
> +                       struct page *page = pfn_to_page(pfn);

If the pfn here is correct, can you not just have something like:

void xen_dma_unmap_page(struct device *hwdev, dma_addr_t handle,
		size_t size, enum dma_data_direction dir,
		struct dma_attrs *attrs)

{
	unsigned long pfn = handle >> PAGE_SHIFT;	/* could use some macros */
	if (!pfn_valid(pfn)) {
		/* FIXME */
		return;
	}
	__generic_dma_ops(hwdev)->unmap_page(hwdev, handle, size, dir, attrs);
}

-- 
Catalin
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