linux-kernel - Re: [PATCH v3 3/7] iommu/vt-d: Always reserve a domain ID for identity setup

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Message-ID: <59eb2544-1c85-4fde-87be-4d97e1f0a246@linux.intel.com>
Date: Wed, 7 Aug 2024 14:19:57 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: baolu.lu@...ux.intel.com, Joerg Roedel <joro@...tes.org>,
 Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>,
 Kevin Tian <kevin.tian@...el.com>, iommu@...ts.linux.dev,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 3/7] iommu/vt-d: Always reserve a domain ID for
 identity setup

On 2024/8/7 1:06, Jason Gunthorpe wrote:
> On Tue, Aug 06, 2024 at 10:39:37AM +0800, Lu Baolu wrote:
>> We will use a global static identity domain. Reserve a static domain ID
>> for it.
>>
>> Signed-off-by: Lu Baolu<baolu.lu@...ux.intel.com>
>> ---
>>   drivers/iommu/intel/iommu.c | 6 +++---
>>   1 file changed, 3 insertions(+), 3 deletions(-)
>>
>> diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
>> index 723ea9f3f501..c019fb3b3e78 100644
>> --- a/drivers/iommu/intel/iommu.c
>> +++ b/drivers/iommu/intel/iommu.c
>> @@ -1440,10 +1440,10 @@ static int iommu_init_domains(struct intel_iommu *iommu)
>>   	 * entry for first-level or pass-through translation modes should
>>   	 * be programmed with a domain id different from those used for
>>   	 * second-level or nested translation. We reserve a domain id for
>> -	 * this purpose.
>> +	 * this purpose. This domain id is also used for identity domain
>> +	 * in legacy mode.
>>   	 */
>> -	if (sm_supported(iommu))
>> -		set_bit(FLPT_DEFAULT_DID, iommu->domain_ids);
>> +	set_bit(FLPT_DEFAULT_DID, iommu->domain_ids);
> That should probablyturn into an IDA someday, it would likely be more
> memory efficient than bitmap_zalloc()

I have tried to. But I failed to find a suitable ida interface to
calculate the count of allocated domain IDs to replace bitmap_weight()
in below code.

static ssize_t domains_used_show(struct device *dev,
                                  struct device_attribute *attr, char *buf)
{
         struct intel_iommu *iommu = dev_to_intel_iommu(dev);
         return sysfs_emit(buf, "%d\n",
                           bitmap_weight(iommu->domain_ids,
                                         cap_ndoms(iommu->cap)));
}
static DEVICE_ATTR_RO(domains_used);

Thanks,
baolu