Re: [Qemu-devel] [PATCH 13/13] iommu: Add a memory barrier to DMA RW function

[Anthony Liguori <anthony@codemonkey.ws>]

On 05/09/2012 11:49 PM, Benjamin Herrenschmidt wrote:
> From: David Gibson<david@gibson.dropbear.id.au>
>
> The emulated devices can run simultaneously with the guest, so
> we need to be careful with ordering of load and stores done by
> them to the guest system memory, which need to be observed in
> the right order by the guest operating system.
>
> The simplest way for now to address that is to stick a memory
> barrier in the main DMA read/write function of the iommu layer,
> this will make everything using that layer hopefully "just work".
>
> We don't emulate devices supporting the relaxed ordering PCIe
> feature nor do we want to look at doing more fine grained
> barriers for now as it could quickly become too complex and not
> worth the cost.
>
> Note that this will not help devices using the map/unmap APIs,
> those will need to use explicit barriers, similar to what
> virtio does.
>
> Signed-off-by: David Gibson<david@gibson.dropbear.id.au>
> Signed-off-by: Benjamin Herrenschmidt<benh@kernel.crashing.org>
> ---
>   dma-helpers.c |    3 +++
>   1 file changed, 3 insertions(+)
>
> diff --git a/dma-helpers.c b/dma-helpers.c
> index 36fa963..4350cdf 100644
> --- a/dma-helpers.c
> +++ b/dma-helpers.c
> @@ -312,6 +312,9 @@ int iommu_dma_memory_rw(DMAContext *dma, dma_addr_t addr,
>           buf += plen;
>       }
>
> +    /* HACK: full memory barrier here */
> +    __sync_synchronize();

I thought you were going to limit this to the TCE iommu?

Regards,

Anthony Liguori

>       return 0;
>   }
>