On Sat, Oct 20, 2018 at 03:36:54PM +0800, Zhen Lei wrote:
> The standard GITS_TRANSLATER register in ITS is only 4 bytes, but
> Hisilicon expands the next 4 bytes to carry some IMPDEF information. That
> means, total 8 bytes data will be written to MSIAddress each time.
>
> MSIAddr: |----4bytes----|----4bytes----|
> | MSIData | IMPDEF |
>
> There is no problem for ITS, because the next 4 bytes space is reserved
> in ITS. But it will overwrite the 4 bytes memory following "sync_count".
> It's very fortunately that the previous and the next neighbour of the
> "sync_count" are both aligned by 8 bytes, so no problem is met now.
>
> It's good to explicitly add a workaround:
> 1. Add gcc __attribute__((aligned(8))) to make sure that "sync_count" is
> always aligned by 8 bytes.
> 2. Add a "int" struct member to make sure the 4 bytes padding is always
> exist.
>
> There is no functional change.
>
> Signed-off-by: Zhen Lei <[email protected]>
> ---
> drivers/iommu/arm-smmu-v3.c | 15 ++++++++++++++-
> 1 file changed, 14 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
> index 5059d09..624fdd0 100644
> --- a/drivers/iommu/arm-smmu-v3.c
> +++ b/drivers/iommu/arm-smmu-v3.c
> @@ -586,7 +586,20 @@ struct arm_smmu_device {
>
> struct arm_smmu_strtab_cfg strtab_cfg;
>
> - u32 sync_count;
> + /*
> + * The alignment and padding is required by Hi16xx of Hisilicon.
> + * Because the ITS hardware on Hi16xx will truncate the MSIAddress(Here
> + * it's the address of "sync_count") to 8 bytes boundary first, then
> + * write 32 bits MSIdata at offset 0, and 32 bits IMPDEF data at offset
> + * 4. Without this workaround, the adjacent member maybe overwritten.
> + *
> + * |---4bytes---|---4bytes---|
> + * MSIAddress & (~0x7): MSIdata | IMPDEF data|
> + */
> + struct {
> + u32 sync_count;
> + int padding;
> + } __attribute__((aligned(8)));
I thought the conclusion after reviewing your original patch was to maintain
the union and drop the alignment directive? e.g.
union {
u32 sync_count;
u64 padding; /* Hi16xx writes an extra 32 bits of goodness
*/
};
Will
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