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

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Message-ID: <bf0a3891-f7b5-4a19-a86d-c115ab2d6b88@linux.intel.com>
Date: Wed, 11 Sep 2024 15:50:59 +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 2024/9/11 15:20, Nicolin Chen wrote:
> On Wed, Sep 11, 2024 at 06:25:16AM +0000, Tian, Kevin wrote:
>>> From: Jason Gunthorpe<jgg@...dia.com>
>>> Sent: Friday, September 6, 2024 2:22 AM
>>>
>>> On Thu, Sep 05, 2024 at 11:00:49AM -0700, Nicolin Chen wrote:
>>>> On Thu, Sep 05, 2024 at 01:20:39PM -0300, Jason Gunthorpe wrote:
>>>>> On Tue, Aug 27, 2024 at 09:59:54AM -0700, Nicolin Chen wrote:
>>>>>
>>>>>> +static int arm_smmu_viommu_cache_invalidate(struct
>>> iommufd_viommu *viommu,
>>>>>> +                                           struct iommu_user_data_array
>>> *array)
>>>>>> +{
>>>>>> +       struct iommu_domain *domain =
>>> iommufd_viommu_to_parent_domain(viommu);
>>>>>> +
>>>>>> +       return __arm_smmu_cache_invalidate_user(
>>>>>> +                       to_smmu_domain(domain), viommu, array);
>>>>> I'd like to have the viommu struct directly hold the VMID. The nested
>>>>> parent should be sharable between multiple viommus, it doesn't make
>>>>> any sense that it would hold the vmid.
>>>>>
>>>>> This is struggling because it is trying too hard to not have the
>>>>> driver allocate the viommu, and I think we should just go ahead and do
>>>>> that. Store the vmid, today copied from the nesting parent in the vmid
>>>>> private struct. No need for iommufd_viommu_to_parent_domain(), just
>>>>> rework the APIs to pass the vmid down not a domain.
>>>> OK. When I designed all this stuff, we still haven't made mind
>>>> about sharing the s2 domain, i.e. moving the VMID, which might
>>>> need a couple of more patches to achieve.
>>> Yes, many more patches, and don't try to do it now.. But we can copy
>>> the vmid from the s2 and place it in the viommu struct during
>>> allocation time.
>>>
>> does it assume that a viommu object cannot span multiple physical
>> IOMMUs so there is only one vmid per viommu?
> I think so. One the reasons of introducing vIOMMU is to maintain
> the shareability across physical IOMMUs at the s2 HWPT_PAGING.

My understanding of VMID is something like domain id in x86 arch's. Is
my understanding correct?

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?

Thanks,
baolu