linux-kernel - Re: [PATCH v2 17/19] iommu/arm-smmu-v3: Add arm_smmu_viommu_cache

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Message-ID: <cf4fe15c-8bcb-4132-a1fd-b2c8ddf2731b@linux.intel.com>
Date: Thu, 12 Sep 2024 12:45:16 +0800
From: Baolu Lu <baolu.lu@...ux.intel.com>
To: Nicolin Chen <nicolinc@...dia.com>, "Tian, Kevin" <kevin.tian@...el.com>
Cc: baolu.lu@...ux.intel.com, Jason Gunthorpe <jgg@...dia.com>,
 "will@...nel.org" <will@...nel.org>, "joro@...tes.org" <joro@...tes.org>,
 "suravee.suthikulpanit@....com" <suravee.suthikulpanit@....com>,
 "robin.murphy@....com" <robin.murphy@....com>,
 "dwmw2@...radead.org" <dwmw2@...radead.org>,
 "shuah@...nel.org" <shuah@...nel.org>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
 "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
 "linux-arm-kernel@...ts.infradead.org"
 <linux-arm-kernel@...ts.infradead.org>,
 "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
 "eric.auger@...hat.com" <eric.auger@...hat.com>,
 "jean-philippe@...aro.org" <jean-philippe@...aro.org>,
 "mdf@...nel.org" <mdf@...nel.org>, "mshavit@...gle.com"
 <mshavit@...gle.com>,
 "shameerali.kolothum.thodi@...wei.com"
 <shameerali.kolothum.thodi@...wei.com>,
 "smostafa@...gle.com" <smostafa@...gle.com>, "Liu, Yi L" <yi.l.liu@...el.com>
Subject: Re: [PATCH v2 17/19] iommu/arm-smmu-v3: Add
 arm_smmu_viommu_cache_invalidate

On 9/12/24 5:08 AM, Nicolin Chen wrote:
> On Wed, Sep 11, 2024 at 08:17:23AM +0000, Tian, Kevin wrote:
> 
>>> My understanding of VMID is something like domain id in x86 arch's. Is
>>> my understanding correct?
>> yes
>>
>>> If a VMID for an S2 hwpt is valid on physical IOMMU A but has already
>>> been allocated for another purpose on physical IOMMU B, how can it be
>>> shared across both IOMMUs? Or the VMID is allocated globally?
>>>
>> I'm not sure that's a problem. The point is that each vIOMMU object
>> will get a VMID from the SMMU which it's associated to (assume
>> one vIOMMU cannot span multiple SMMU). Whether that VMID
>> is globally allocated or per-SMMU is the policy in the SMMU driver.
>>
>> It's the driver's responsibility to ensure not using a conflicting VMID
>> when creating an vIOMMU instance.
> It can happen to be the same VMID across all physical SMMUs, but
> not necessary to be the same, i.e. two SMMUs might have two VMIDs
> with different ID values, allocated from the their own VMID pools,
> since cache entries in their own TLB can be tagged with their own
> VMIDs.
> 
> Does domain id for intel-iommu have to be the same?

No. A paging domain may have different domain IDs on different IOMMUs
for Intel iommu driver.

> I recall there
> is only one iommu instance on intel chips at this moment?

No. There might be multiple iommu instances on a chip but they share a
common iommu driver ops.

Thanks,
baolu