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

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Message-ID: <d70c2de2-c0c9-49cb-bbc7-773fd615067f@linux.intel.com>
Date: Wed, 7 Aug 2024 21:38:02 +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 20:09, Jason Gunthorpe wrote:
> On Wed, Aug 07, 2024 at 02:19:57PM +0800, Baolu Lu wrote:
>> 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.
> For something debugging like that just use
>    idr_for_each_entry()
>       count++;

Yeah! It works.

> It is a weird thing to put in sysfs in the first place...

Agreed. But it has been there for years. :-)

Thanks,
baolu