linux-kernel - Re: [RFC PATCH 2/7] swiotlb: Make io_tlb_start a physical address instead of a virtual address

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Date:	Tue, 9 Oct 2012 12:43:45 -0400
From:	Konrad Rzeszutek Wilk <konrad@...nel.org>
To:	Alexander Duyck <alexander.h.duyck@...el.com>
Cc:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>, tglx@...utronix.de,
	mingo@...hat.com, hpa@...or.com, rob@...dley.net,
	akpm@...ux-foundation.org, joerg.roedel@....com,
	bhelgaas@...gle.com, shuahkhan@...il.com,
	linux-kernel@...r.kernel.org, devel@...uxdriverproject.org,
	x86@...nel.org
Subject: Re: [RFC PATCH 2/7] swiotlb: Make io_tlb_start a physical address
 instead of a virtual address

On Thu, Oct 04, 2012 at 01:22:58PM -0700, Alexander Duyck wrote:
> On 10/04/2012 10:19 AM, Konrad Rzeszutek Wilk wrote:
> >>>> @@ -450,7 +451,7 @@ void *swiotlb_tbl_map_single(struct device *hwdev, dma_addr_t tbl_dma_addr,
> >>>>  				io_tlb_list[i] = 0;
> >>>>  			for (i = index - 1; (OFFSET(i, IO_TLB_SEGSIZE) != IO_TLB_SEGSIZE - 1) && io_tlb_list[i]; i--)
> >>>>  				io_tlb_list[i] = ++count;
> >>>> -			dma_addr = io_tlb_start + (index << IO_TLB_SHIFT);
> >>>> +			dma_addr = (char *)phys_to_virt(io_tlb_start) + (index << IO_TLB_SHIFT);
> >>> I think this is going to fall flat with the other user of
> >>> swiotlb_tbl_map_single - Xen SWIOTLB. When it allocates the io_tlb_start
> >>> and does it magic to make sure its under 4GB - the io_tlb_start swath
> >>> of memory, ends up consisting of 2MB chunks of contingous spaces. But each
> >>> chunk is not linearly in the DMA space (thought it is in the CPU space).
> >>>
> >>> Meaning the io_tlb_start region 0-2MB can fall within the DMA address space
> >>> of 2048MB->2032MB, and io_tlb_start offset 2MB->4MB, can fall within 1024MB->1026MB,
> >>> and so on (depending on the availability of memory under 4GB).
> >>>
> >>> There is a clear virt_to_phys(x) != virt_to_dma(x).
> >> Just to be sure I understand you are talking about DMA address space,
> >> not physical address space correct?  I am fully aware that DMA address
> >> space can be all over the place.  When I was writing the patch set the
> >> big reason why I decided to stop at physical address space was because
> >> DMA address spaces are device specific.
> >>
> >> I understand that virt_to_phys(x) != virt_to_dma(x) for many platforms,
> >> however that is not my assertion.  My assertion is (virt_to_phys(x) + y)
> >> == virt_to_phys(x + y).  This should be true for any large block of
> >> contiguous memory that is DMA accessible since the CPU and the device
> >> should be able to view the memory in the same layout.  If that wasn't
> > That is true mostly for x86 but not all platforms do this.
> >
> >> true I don't think is_swiotlb_buffer would be working correctly since it
> >> is essentially operating on the same assumption prior to my patches.
> > There are two pieces here - the is_swiotlb_buffer and the swiotlb_tbl_[map|unmap]
> > functions.
> >
> > The is_swiotlb_buffer is operating on that principle (and your change
> > to reflect that is OK). The swiotlb_tbl_[*] is not.
> >> If you take a look at patches 4 and 5 I do address changes that end up
> >> needing to be made to Xen SWIOTLB since it makes use of
> >> swiotlb_tbl_map_single.  All that I effectively end up changing is that
> >> instead of messing with a void pointer we instead are dealing with a
> >> physical address, and instead of calling xen_virt_to_bus we end up
> >> calling xen_phys_to_bus and thereby drop one extra virt_to_phys call in
> >> the process.
> > Sure that is OK. All of those changes when we bypass the bounce
> > buffer look OK (thought I should double-check again the patch to make
> > sure and also just take it for a little test spin).
> 
> I'm interesting in finding out what the results of your test spin are. 

Haven't gotten to that yet :-(
> 
> > The issue is when we do _use_ the bounce buffer. At that point we
> > run into the allocation from the bounce buffer where the patches
> > assume that the 64MB swath of bounce buffer memory is bus (or DMA)
> > memory contingous. And that is not the case sadly.
> 
> I think I understand what you are saying now.  However, I don't think
> the issue applies to my patches.

Great.
> 
> If I am not mistaken what you are talking about is the pseudo-physical
> memory versus machine memory.  I understand the 64MB block is not
> machine-memory contiguous, but it should be pseudo-physical contiguous
> memory.  As such using the pseudo-physical addresses instead of virtual
> addresses should function the same way as using true physical addresses
> to replace virtual addresses.  All of the physical memory translation to
> machine memory translation is happening in xen_phys_to_bus and all of
> the changes I have made take place before that so the bounce buffers
> should still be working correctly.  In addition none of the changes I

OK.
> have made change the bounce buffer boundary assumptions so we should
> have no bounce buffers mapped across the 2MB boundaries.
> 
> Thanks,
> 
> Alex
> 
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