linux-kernel - Re: [PATCH v2 1/4] iommu/vt-d: Check before setting PGSNP bit in pasid table entry

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Date:   Fri, 22 Apr 2022 21:04:11 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>
Cc:     baolu.lu@...ux.intel.com,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 1/4] iommu/vt-d: Check before setting PGSNP bit in
 pasid table entry

On 2022/4/22 10:47, Tian, Kevin wrote:
>> From: Lu Baolu
>> Sent: Thursday, April 21, 2022 7:36 PM
>>
>> The latest VT-d specification states that the PGSNP field in the pasid
>> table entry should be treated as Reserved(0) for implementations not
>> supporting Snoop Control (SC=0 in the Extended Capability Register).
>> This adds a check before setting the field.
>>
>> Signed-off-by: Lu Baolu<baolu.lu@...ux.intel.com>
>> ---
>>   drivers/iommu/intel/pasid.c | 13 ++++++++++---
>>   1 file changed, 10 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/iommu/intel/pasid.c b/drivers/iommu/intel/pasid.c
>> index f8d215d85695..5cb2daa2b8cb 100644
>> --- a/drivers/iommu/intel/pasid.c
>> +++ b/drivers/iommu/intel/pasid.c
>> @@ -625,8 +625,14 @@ int intel_pasid_setup_first_level(struct intel_iommu
>> *iommu,
>>   		}
>>   	}
>>
>> -	if (flags & PASID_FLAG_PAGE_SNOOP)
>> -		pasid_set_pgsnp(pte);
>> +	if (flags & PASID_FLAG_PAGE_SNOOP) {
>> +		if (ecap_sc_support(iommu->ecap)) {
>> +			pasid_set_pgsnp(pte);
>> +		} else {
>> +			pasid_clear_entry(pte);
>> +			return -EINVAL;
>> +		}
>> +	}
>>
>>   	pasid_set_domain_id(pte, did);
>>   	pasid_set_address_width(pte, iommu->agaw);
>> @@ -710,7 +716,8 @@ int intel_pasid_setup_second_level(struct
>> intel_iommu *iommu,
>>   	pasid_set_fault_enable(pte);
>>   	pasid_set_page_snoop(pte, !!ecap_smpwc(iommu->ecap));
>>
>> -	if (domain->domain.type == IOMMU_DOMAIN_UNMANAGED)
>> +	if (ecap_sc_support(iommu->ecap) &&
>> +	    domain->domain.type == IOMMU_DOMAIN_UNMANAGED)
>>   		pasid_set_pgsnp(pte);
>>
> This should be rebased on top of Jason's enforce coherency series
> instead of blindly setting it. No matter whether it's legacy mode
> where we set SNP in PTE or scalable mode where we set PGSNP
> in PASID entry for entire page table, the trigger point should be
> same i.e. when someone calls enforce_cache_coherency().

With Jason's enforce coherency series merged, we even don't need to set
PGSNP bit of a pasid entry for second level translation. 2nd level
always supports SNP in PTEs, so set PGSNP in pasid table entry is
unnecessary.

Any thoughts?

Best regards,
baolu