Subject: [RFC PATCH 05/30] iommu/arm-smmu-v3: Disable tagged pointers when ATS is in use

From: [email-masked] (Leizhen (ThunderTown))
Date: Mon, 22 May 2017 14:27:35 +0800

On 2017/2/28 3:54, Jean-Philippe Brucker wrote:
> The ARM architecture has a "Top Byte Ignore" (TBI) option that makes the
> MMU mask out bits [63:56] of an address, allowing a userspace application
> to store data in its pointers.
> 
> The ATS doesn't have an architected mechanism to enable TBI, and might
> create ATC entries for addresses that include a tag. Software would then
> have to send ATC invalidation packets for each 255 possible alias of an
> address, or just wipe the whole address space. This is not a viable
> option, so disable TBI when ATS is in use.
> 
> It is unclear for the moment how this restriction will affect user
> applications. One example I can imagine (with my complete lack of
> knowledge about JIT engines and their use of tagged pointers) is a JIT
> translating a WebCL applications that uses SVM. Since this kind of
> interpreted language doesn't expose addresses, the interpreter and SVM
> implementations will be given the opportunity to do the right thing and
> remove tags before handing pointers to devices.
> 
> Ideally we should remove TBI only for domains that are susceptible to use
> ATS. But at the point we're writing the context descriptor of a domain, we
> are still probing the first device in the group. If that device doesn't
> have an ATC but the next one does, we would then have to clear the TBI
> bit, invalidate the ATCs of previous devices, and notify the drivers that
> their devices cannot use tagged pointers anymore. Such a level of
> complexity doesn't make any sense here since it is unlikely devices will
> use tagged pointers at all.
> 
> Signed-off-by: Jean-Philippe Brucker <[email-masked]>
> ---
>  drivers/iommu/arm-smmu-v3.c | 7 ++++---
>  1 file changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/iommu/arm-smmu-v3.c b/drivers/iommu/arm-smmu-v3.c
> index e7b940146ae3..06b29d4fcf65 100644
> --- a/drivers/iommu/arm-smmu-v3.c
> +++ b/drivers/iommu/arm-smmu-v3.c
> @@ -987,7 +987,7 @@ static void arm_smmu_cmdq_issue_cmd(struct arm_smmu_device *smmu,
>  }
>  
>  /* Context descriptor manipulation functions */
> -static u64 arm_smmu_cpu_tcr_to_cd(u64 tcr)
> +static u64 arm_smmu_cpu_tcr_to_cd(struct arm_smmu_device *smmu, u64 tcr)
>  {
>  	u64 val = 0;
>  
> @@ -1000,7 +1000,8 @@ static u64 arm_smmu_cpu_tcr_to_cd(u64 tcr)
>  	val |= ARM_SMMU_TCR2CD(tcr, EPD0);
>  	val |= ARM_SMMU_TCR2CD(tcr, EPD1);
>  	val |= ARM_SMMU_TCR2CD(tcr, IPS);
> -	val |= ARM_SMMU_TCR2CD(tcr, TBI0);
> +	if (!(smmu->features & ARM_SMMU_FEAT_ATS))
> +		val |= ARM_SMMU_TCR2CD(tcr, TBI0);
Maybe we should always disable TBI. Otherwise, a device behind a ATS supported or
non-supported SMMU should use different strategies.

>  
>  	return val;
>  }
> @@ -1014,7 +1015,7 @@ static void arm_smmu_write_ctx_desc(struct arm_smmu_device *smmu,
>  	 * We don't need to issue any invalidation here, as we'll invalidate
>  	 * the STE when installing the new entry anyway.
>  	 */
> -	val = arm_smmu_cpu_tcr_to_cd(cfg->cd.tcr) |
> +	val = arm_smmu_cpu_tcr_to_cd(smmu, cfg->cd.tcr) |
>  #ifdef __BIG_ENDIAN
>  	      CTXDESC_CD_0_ENDI |
>  #endif
> 

-- 
Thanks!
BestRegards