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

[Stefano Stabellini <stefano.stabellini@xxxxxxxxxxxxx>]

On Fri, 17 Jan 2014, 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.

We are lucky that LPAE only supports 40 bits otherwise we would need to
change any other functions that use unsigned long for mfn, starting from
set_phys_to_machine.


> 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.
> 
> 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;

I'd add this comment:

/* avoid PFN_PHYS because phys_addr_t can be 32bit when dma_addr_t is
 * 64bit leading to a loss in information if the shift is done before
 * casting to 64bit. */

> +     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 */

This check is useless because PFN_PHYS contains a cast to (phys_addr_t)
that is 32 bit. I think you'll have to:

dma_addr_t dma = ((dma_addr_t)mfn_to_pfn(PFN_DOWN(baddr))) << PAGE_SHIFT;


> +     paddr |= baddr & ~PAGE_MASK;
> +
> +     return paddr;
>  }


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