Re: [Xen-devel] [PATCH] xen: swiotlb: handle sizeof(dma_addr_t) != sizeof(phys_addr_t)

[Ian Campbell <Ian.Campbell@xxxxxxxxxx>]

On Tue, 2014-01-21 at 17:03 -0500, Konrad Rzeszutek Wilk wrote:

(nb: I posted a v3 at
http://article.gmane.org/gmane.linux.ports.arm.kernel/295594
)

> On Fri, Jan 17, 2014 at 05:24:53PM +0000, Ian Campbell wrote:
> > The use of phys_to_machine and machine_to_phys in the phys<=>bus conversions
> > causes us to lose the top bits of the DMA address if the size of a DMA 
> > address is not the same as the size of the phyiscal address.
> > 
> > This can happen in practice on ARM where foreign pages can be above 4GB even
> > though the local kernel does not have LPAE page tables enabled (which is
> > totally reasonable if the guest does not itself have >4GB of RAM). In this
> > case the kernel still maps the foreign pages at a phys addr below 4G (as it
> > must) but the resulting DMA address (returned by the grant map operation) is
> > much higher.
> > 
> > This is analogous to a hardware device which has its view of RAM mapped up
> > high for some reason.
> > 
> > This patch makes I/O to foreign pages (specifically blkif) work on 32-bit 
> > ARM
> > systems with more than 4GB of RAM.
> 
> There was another patch posted by somebody from Citrix for a fix on 
> 32-bit x86 dom0 with more than 4GB of RAM (for x86 platforms).
> 
> Their fix was in the generic parts of code. Changing most of the 'unsigned'
> to 'phys_addr_t' or such. Is his patch better or will this patch replace his?

I believe they are orthogonal, or at least I'm not (yet) hitting the
same issue as Stefano P, the alloc cohoerent code paths are not involved
in the issue I'm seeing because it involves foreign pages whose
MFN/dma_addr is very high, not DMA to devices which are up high.

Ian.

> It was " dma-mapping: dma_alloc_coherent_mask return dma_addr_t"
> https://lkml.org/lkml/2013/12/10/593
> 
> > 
> > Signed-off-by: Ian Campbell <ian.campbell@xxxxxxxxxx>
> > ---
> >  arch/arm/Kconfig          |    1 +
> >  drivers/xen/swiotlb-xen.c |   14 ++++++++++++--
> >  2 files changed, 13 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> > index c1f1a7e..24307dc 100644
> > --- a/arch/arm/Kconfig
> > +++ b/arch/arm/Kconfig
> > @@ -1885,6 +1885,7 @@ config XEN
> >     depends on !GENERIC_ATOMIC64
> >     select ARM_PSCI
> >     select SWIOTLB_XEN
> > +   select ARCH_DMA_ADDR_T_64BIT
> >     help
> >       Say Y if you want to run Linux in a Virtual Machine on Xen on ARM.
> >  
> > diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
> > index 1eac073..b626c79 100644
> > --- a/drivers/xen/swiotlb-xen.c
> > +++ b/drivers/xen/swiotlb-xen.c
> > @@ -77,12 +77,22 @@ static u64 start_dma_addr;
> >  
> >  static inline dma_addr_t xen_phys_to_bus(phys_addr_t paddr)
> >  {
> > -   return phys_to_machine(XPADDR(paddr)).maddr;
> > +   unsigned long mfn = pfn_to_mfn(PFN_DOWN(paddr));
> > +   dma_addr_t dma = (dma_addr_t)mfn << PAGE_SHIFT;
> > +   dma |= paddr & ~PAGE_MASK;
> > +   return dma;
> >  }
> >  
> >  static inline phys_addr_t xen_bus_to_phys(dma_addr_t baddr)
> >  {
> > -   return machine_to_phys(XMADDR(baddr)).paddr;
> > +   dma_addr_t dma = PFN_PHYS(mfn_to_pfn(PFN_DOWN(baddr)));
> > +   phys_addr_t paddr = dma;
> > +
> > +   BUG_ON(paddr != dma); /* truncation has occurred, should never happen */
> > +
> > +   paddr |= baddr & ~PAGE_MASK;
> > +
> > +   return paddr;
> >  }
> >  
> >  static inline dma_addr_t xen_virt_to_bus(void *address)
> > -- 
> > 1.7.10.4
> > 



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