Re: [PATCH] iommu/amd: Fix unity mapping initialization race

Wan Zongshun
2016-07-10T11:41:00Z

On 2016年07月07日 00:00, Joerg Roedel wrote:
> From: Joerg Roedel <jroedel@suse.de>
>
> There is a race condition in the AMD IOMMU init code that
> causes requested unity mappings to be blocked by the IOMMU
> for a short period of time. This results on boot failures
> and IO_PAGE_FAULTs on some machines.
>
> Fix this by making sure the unity mappings are installed
> before all other DMA is blocked.
>
> Fixes: aafd8ba0ca74 ('iommu/amd: Implement add_device and remove_device')
> Cc: stable@vger.kernel.org # v4.2+
> Signed-off-by: Joerg Roedel <jroedel@suse.de>
> ---
> drivers/iommu/amd_iommu_init.c | 14 ++++++++++++--
> 1 file changed, 12 insertions(+), 2 deletions(-)
>
> diff --git a/drivers/iommu/amd_iommu_init.c b/drivers/iommu/amd_iommu_init.c
> index d091def..59741ea 100644
> --- a/drivers/iommu/amd_iommu_init.c
> +++ b/drivers/iommu/amd_iommu_init.c
> @@ -1568,13 +1568,23 @@ static int __init amd_iommu_init_pci(void)
> break;
> }
>
> + /*
> + * Order is important here to make sure any unity map requirements are
> + * fulfilled. The unity mappings are created and written to the device
> + * table during the amd_iommu_init_api() call.
> + *
> + * After that we call init_device_table_dma() to make sure any
> + * uninitialized DTE will block DMA, and in the end we flush the caches
> + * of all IOMMUs to make sure the changes to the device table are
> + * active.
> + */

Joerg,

Do you mean we need enable the V and TV bits to DTE entry after all DTEs
tables were initialized completely?

I checked this function 'init_device_table_dma', and find it just set
V and TV bit, to set translation info valid and DTE bits127:1 valid.

So I just think all things it should to do are to allow DMA access,
GPA-to-SPA translation should be active, why you add function comments
below is to not allow DMA access and suppress all page faults?

/*
* Init the device table to not allow DMA access for devices and
* suppress all page faults
*/
static void init_device_table_dma(void)

> + ret = amd_iommu_init_api();
> +
> init_device_table_dma();
>
> for_each_iommu(iommu)
> iommu_flush_all_caches(iommu);
>
> - ret = amd_iommu_init_api();
> -
> if (!ret)
> print_iommu_info();
>
>