Re: [RFC V1 02/12] iommufd: no DMA to BARs

[Yi Liu]

On 2024/7/21 03:15, Steve Sistare wrote:
> Do not map VFIO PCI BARs for DMA.  This stops a raft of warnings of the
> following form at QEMU start time when using -object iommufd:
>
> qemu-kvm: warning: IOMMU_IOAS_MAP failed: Bad address, PCI BAR?
> qemu-kvm: vfio_container_dma_map(0x555558282db0, 0x8800010000, 0x4000, 
> 0x7ffff7ff0000) = -14 (Bad address)

It is required as Alex pointed, so no need to pay further attempt to hide
this message. And there were efforts to make it. But not done yet. Below
links may be helpful if you are interested about the history.

[1] https://lore.kernel.org/kvm/14-v4-0de2f6c78ed0+9d1-iommufd_jgg@nvidia.com/
[2] https://lore.kernel.org/kvm/20240624141139.GH29266@unreal/
[3] 
https://lore.kernel.org/kvm/0-v1-9e6e1739ed95+5fa-vfio_dma_buf_jgg@nvidia.com/

> Signed-off-by: Steve Sistare <steven.sistare@oracle.com>
> ---
>   hw/vfio/common.c      | 3 ++-
>   hw/vfio/helpers.c     | 1 +
>   include/exec/memory.h | 1 +
>   3 files changed, 4 insertions(+), 1 deletion(-)
>
> diff --git a/hw/vfio/common.c b/hw/vfio/common.c
> index da2e0ec..403d45a 100644
> --- a/hw/vfio/common.c
> +++ b/hw/vfio/common.c
> @@ -248,7 +248,8 @@ static bool 
> vfio_listener_skipped_section(MemoryRegionSection *section)
>               * are never accessed by the CPU and beyond the address width of
>               * some IOMMU hardware.  TODO: VFIO should tell us the IOMMU width.
>               */
> -           section->offset_within_address_space & (1ULL << 63);
> +           section->offset_within_address_space & (1ULL << 63) ||
> +           section->mr->no_dma;
>   }
>   
>   /* Called with rcu_read_lock held.  */
> diff --git a/hw/vfio/helpers.c b/hw/vfio/helpers.c
> index b14edd4..e4cfdd2 100644
> --- a/hw/vfio/helpers.c
> +++ b/hw/vfio/helpers.c
> @@ -435,6 +435,7 @@ int vfio_region_mmap(VFIORegion *region)
>                                             memory_region_owner(region->mem),
>                                             name, region->mmaps[i].size,
>                                             region->mmaps[i].mmap);
> +        region->mmaps[i].mem.no_dma = true;
>           g_free(name);
>           memory_region_add_subregion(region->mem, region->mmaps[i].offset,
>                                       &region->mmaps[i].mem);
> diff --git a/include/exec/memory.h b/include/exec/memory.h
> index ea03ef2..850cc8c 100644
> --- a/include/exec/memory.h
> +++ b/include/exec/memory.h
> @@ -794,6 +794,7 @@ struct MemoryRegion {
>       bool unmergeable;
>       uint8_t dirty_log_mask;
>       bool is_iommu;
> +    bool no_dma;
>       RAMBlock *ram_block;
>       Object *owner;
>       /* owner as TYPE_DEVICE. Used for re-entrancy checks in MR access hotpath 
> */

--
Regards,
Yi Liu