[Xen-devel] Re: [PATCH] x86: define arch_vm_get_page_prot to set _PAGE_IOMAP on VM_IO vmas

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Fri, Oct 22, 2010 at 02:02:31PM -0400, Konrad Rzeszutek Wilk wrote:
> On Fri, Oct 22, 2010 at 09:44:08AM -0700, H. Peter Anvin wrote:
> > On 10/22/2010 08:08 AM, Konrad Rzeszutek Wilk wrote:
> > >>
> > >> Okay, could you clarify this part a bit?  Why does the kernel need to
> > >> know the difference between "pseudo-physical" and "machine addresses" at
> > >> all?  If they overlap, there is a problem, and if they don't overlap, it
> > >> will be a 1:1 mapping anyway...
> > > 
> > > The flag (_PAGE_IOMAP) is used when we set the PTE so that the MFN value 
> > > is
> > > used instead of the PFN. We need that b/c when a driver does page_to_pfn()
> > > it ends up using the PFN as bus address to write out registers data.
> > > 
> > > Without this patch, the page->virt->PFN value is used and the PFN != to 
> > > real MFN
> > > so we end up writing in a memory address that the PCI device has no idea 
> > > about.
> > > By setting the PTE with the MFN, the virt->PFN gets the real MFN value.
> > > 
> > > The drivers I am talking about are mostly, if not all, located in 
> > > drivers/gpu
> > > and it looks that we are missing two more patches to utilize the patch
> > > that Jeremy posted.
> > > 
> > > Please note that I am _not_ suggesting that the two patches
> > > below should go out - I still need to post them on drm mailing list.
> > > 
> > 
> > I'm still seriously confused.  If I understand this correctly, we're
> > talking about DMA addresses here (as opposed to PIO addresses, i.e.
> > BARs), right?
> 
> Correct. The BARs are ok since they go through the ioremap.

Whoops. I answered that incorrectly. No, we are not talking about
BARs. The BARs are ok (look at x86/PCI: make sure _PAGE_IOMAP it set on pci 
mappings
patch, http://amailbox.org/mailarchive/linux-kernel/2010/10/12/4630930/thread)

We are talking about the GPU's VM engine (or the GART on the Northbridge).

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