RE: [PATCH] iommu/intel: SNP bit is not dependent on iommu domaincoherency

[Zhang, Yang Z]

Alex Williamson wrote on 2013-12-24:
> David,
> 
> Any comments on this patch?  Thanks,
> 

Hi Alex,

There do have some IOMMUs will treat SNP bit in the PTE as reserved (0) and will cause a reserved field violation fault if it is set but hardware not support snoop-control(bit 7 in ECAP_REG is 0). So your patch seems wrong to me.

> Alex
> 
> On Tue, 2013-10-29 at 10:21 -0600, Alex Williamson wrote:
>> The setting of the SNP bit in the intel-iommu page tables should not
>> be dependent on the current capability of the iommu domain.  The
>> current VT-d spec (2.2) indicates the SNP bit is "treated as
>> reserved[0] by hardware implementations not supporting Snoop Control".
>> Furthermore, section 3.7.3 indicates:
>> 
>>   If the Snoop Control (SC) field in extended capability Register is
>>   reported as 0, snoop behavior for access to the page mapped through
>>   second-level translation is determined by the no-snoop attribute in
>>   the request.
>> This all seems to indicate that hardware incapable of Snoop Control
>> will handle the SNP bit as zero regardless of the value stored in
>> the PTE.
>> 
>> The trouble with the current implementation is that mapping flags
>> depend on the state of the iommu domain at the time of the mapping,
>> yet no attempt is made to update existing mappings when the iommu
>> domain composition changes.  This leaves the iommu domain in a state
>> where some mappings may enforce coherency, others do not, and the
>> user of the IOMMU API has no ability to later enable the desired
>> flags atomically with respect to DMA.
>> 
>> If we always honor the IOMMU_CACHE flag then an IOMMU API user who
>> specifies IOMMU_CACHE for all mappings can assume that the coherency
>> of the mappings within a domain follow the coherency capability of
>> the domain itself.
>> 
>> Signed-off-by: Alex Williamson <alex.williamson@xxxxxxxxxx>
>> ---
>>  drivers/iommu/intel-iommu.c |    2 +-
>>  1 file changed, 1 insertion(+), 1 deletion(-)
>> diff --git a/drivers/iommu/intel-iommu.c
>> b/drivers/iommu/intel-iommu.c index 15e9b57..c46c6a6 100644
>> --- a/drivers/iommu/intel-iommu.c
>> +++ b/drivers/iommu/intel-iommu.c
>> @@ -4084,7 +4084,7 @@ static int intel_iommu_map(struct iommu_domain
> *domain,
>>  		prot |= DMA_PTE_READ;
>>  	if (iommu_prot & IOMMU_WRITE)
>>  		prot |= DMA_PTE_WRITE;
>> -	if ((iommu_prot & IOMMU_CACHE) && dmar_domain->iommu_snooping)
>> +	if (iommu_prot & IOMMU_CACHE)
>>  		prot |= DMA_PTE_SNP;
>>  
>>  	max_addr = iova + size;
> 
>


Best regards,
Yang


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