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

Jean-Philippe Brucker
2017-05-22T14:02:45Z

On 22/05/17 07:27, Leizhen (ThunderTown) wrote:
> 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 <jean-philippe.brucker@arm.com>
>> ---
>> 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.

I don't have any objection to this. Since ATS state shouldn't be visible
to userspace, the responsibility of sanitizing tagged pointers for DMA
already lies on the rare applications that use of them. I haven't thought
about the stall handling code yet, but I'm sure there are creative ways to
break it using tagged pointers, and I'd rather not go down that rabbit hole.

I'll update this patch accordingly in next version.

Thanks,
Jean

>> 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
>>
>