linux-kernel - Re: [Xen-devel] [PATCH 1/1] XEN: Use correct masking in xen_swiotlb_alloc

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Message-ID: <20120904143731.GC23361@phenom.dumpdata.com>
Date:	Tue, 4 Sep 2012 10:37:31 -0400
From:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:	Stefano Panella <stefano.panella@...rix.com>
Cc:	David Vrabel <david.vrabel@...rix.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"xen-devel@...ts.xensource.com" <xen-devel@...ts.xensource.com>
Subject: Re: [Xen-devel] [PATCH 1/1] XEN: Use correct masking in
 xen_swiotlb_alloc_coherent.

On Tue, Sep 04, 2012 at 03:07:42PM +0100, Stefano Panella wrote:
> On 08/31/2012 05:40 PM, Konrad Rzeszutek Wilk wrote:
> >On Fri, Aug 31, 2012 at 01:47:05PM +0100, David Vrabel wrote:
> >>On 31/08/12 10:57, Stefano Panella wrote:
> >>>When running 32-bit pvops-dom0 and a driver tries to allocate a coherent
> >>>DMA-memory the xen swiotlb-implementation returned memory beyond 4GB.
> >>>
> >>>This caused for example not working sound on a system with 4 GB and a 64-bit
> >>>compatible sound-card with sets the DMA-mask to 64bit.
> >>>
> >>>On bare-metal and the forward-ported xen-dom0 patches from OpenSuse a coherent
> >>>DMA-memory is always allocated inside the 32-bit address-range by calling
> >>>dma_alloc_coherent_mask.
> >>We should have the same behaviour under Xen as bare metal so:
> >>
> >>Acked-By: David Vrabel <david.vrabel@...rix.com>
> >>
> >>This does limit the DMA mask to 32-bits by passing it through an
> >>unsigned long, which seems a bit sneaky...
> >so is the issue that we are not casting it from 'u64' to 'u32'
> >(unsigned long) on 32-bit?
> 
> Yes. I do not completely understand why but I think on 32-bit kernel we need to cast dma_mask to u32. This is done automatically using dma_alloc_coherent_mask()
> 
> >
> >>Presumably the sound card is capable of handling 64 bit physical
> >>addresses (or it would break under 64-bit kernels) so it's not clear why
> >>this sound driver requires this restriction.
> >>
> >>Is there a bug in the sound driver or sound subsystem where it's
> >>truncating a dma_addr_t by assigning it to an unsigned long or similar?
> >>
> >>>--- a/drivers/xen/swiotlb-xen.c
> >>>+++ b/drivers/xen/swiotlb-xen.c
> >>>@@ -232,7 +232,7 @@ xen_swiotlb_alloc_coherent(struct device *hwdev, size_t size,
> >>>  		return ret;
> >>>  	if (hwdev && hwdev->coherent_dma_mask)
> >>>-		dma_mask = hwdev->coherent_dma_mask;
> >>>+		dma_mask = dma_alloc_coherent_mask(hwdev, flags);
> >>Suggest
> >>
> >>     if (hwdev)
> >>         dma_mask = dma_alloc_coherent_mask(hwdev, flags)
> 
> I can change the patch like that if you like.
> 
> >Isn't that code just doing this:
> >atic inline unsigned long dma_alloc_coherent_mask(struct device *dev,
> >                                                     gfp_t gfp)
> >{
> >         unsigned long dma_mask = 0;
> >
> >         dma_mask = dev->coherent_dma_mask;
> >         if (!dma_mask)
> >                 dma_mask = (gfp & GFP_DMA) ? DMA_BIT_MASK(24) :
> >DMA_BIT_MASK(32);
> >
> >         return dma_mask;
> >}
> >
> >and in our code, the dma_mask by default is DMA_BIT_MASK(32):
> >
> >u64 dma_mask = DMA_BIT_MASK(32);
> >
> >So what I am missing?
> 
> I am not sure what you mean with "what am I missing?"
> 
> Current code looks like:
> 
> void *
> xen_swiotlb_alloc_coherent(struct device *hwdev, size_t size,
>                            dma_addr_t *dma_handle, gfp_t flags,
>                            struct dma_attrs *attrs)
> {
>         void *ret;
>         int order = get_order(size);
>         u64 dma_mask = DMA_BIT_MASK(32);
>         unsigned long vstart;
>         phys_addr_t phys;
>         dma_addr_t dev_addr;
> 
>         /*
>         * Ignore region specifiers - the kernel's ideas of
>         * pseudo-phys memory layout has nothing to do with the
>         * machine physical layout.  We can't allocate highmem
>         * because we can't return a pointer to it.
>         */
>         flags &= ~(__GFP_DMA | __GFP_HIGHMEM);
> 
>         if (dma_alloc_from_coherent(hwdev, size, dma_handle, &ret))
>                 return ret;
> 
>         vstart = __get_free_pages(flags, order);
>         ret = (void *)vstart;
> 
>         if (!ret)
>                 return ret;
> 
>         if (hwdev && hwdev->coherent_dma_mask)
>                 dma_mask = hwdev->coherent_dma_mask;
> 
> 
> So if hwdev->coherent_dma_mask is set to 0xffffffffffffffff our dma_mask will
> be u64 set to 0xffffffffffffffff even if we set it to DMA_BIT_MASK(32) previously.

That is what I was missing. Let me include that in the git commit and also
put this patch on the stable tree.

> 
> I hope I am not getting this wrong and let me know if I should send an updated version
> of the patch including David V. change.
> 
> Regards,
> 
> Stefano
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