linux-kernel - Re: [PATCH v4 3/6] iommu/sva: Stop using ioasid

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Date:   Sat, 11 Mar 2023 09:18:30 -0800
From:   Fenghua Yu <fenghua.yu@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>
CC:     Baolu Lu <baolu.lu@...ux.intel.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        LKML <linux-kernel@...r.kernel.org>, <iommu@...ts.linux.dev>,
        Joerg Roedel <joro@...tes.org>,
        "Jean-Philippe Brucker" <jean-philippe@...aro.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        X86 Kernel <x86@...nel.org>, <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>, <corbet@....net>,
        <vkoul@...nel.org>, <dmaengine@...r.kernel.org>,
        <linux-doc@...r.kernel.org>, Robin Murphy <robin.murphy@....com>,
        Will Deacon <will@...nel.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Raj Ashok <ashok.raj@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>, Yi Liu <yi.l.liu@...el.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Kirill Shutemov <kirill.shutemov@...ux.intel.com>
Subject: Re: [PATCH v4 3/6] iommu/sva: Stop using ioasid_set for SVA

Hi, Jason and Jacob,

On 3/8/23 10:23, Jason Gunthorpe wrote:
> On Tue, Mar 07, 2023 at 02:32:09PM -0800, Jacob Pan wrote:
>> Hi Jason,
>>
>> On Fri, 3 Mar 2023 09:15:45 -0400, Jason Gunthorpe <jgg@...dia.com> wrote:
>>
>>> On Fri, Mar 03, 2023 at 05:57:41PM +0800, Baolu Lu wrote:
>>>> On 2023/3/3 17:32, Jean-Philippe Brucker wrote:
>>>>>> I suppose the common thing is reserving some kind of special
>>>>>> PASIDs.
>>>>> Are you planning to use RID_PASID != 0 in VT-d?  Otherwise we could
>>>>> just communicate min_pasid from the IOMMU driver the same way we do
>>>>> max_pasid.
>>>>>
>>>>> Otherwise I guess re-introduce a lighter ioasid_alloc() that the IOMMU
>>>>> driver calls to reserve PASID0/RID_PASID.
>>>>
>>>> Yes. We probably will use a non-zero RID_PASID in the future. An
>>>> interface to reserve (or allocate) a PASID from iommu_global_pasid_ida
>>>> should work then.
>>>
>>> Just allowing the driver to store XA_ZERO_ENTRY would be fine
>>>
>> It looks like there are incoming users of iommu_sva_find()
>> https://lore.kernel.org/lkml/20230306163138.587484-1-fenghua.yu@intel.com/T/#m1fc97725a0e56ea269c8bdabacee447070d51846
>> Should we keep the xa here instead of the global ida?
> 
> I'm not sure this should be in the iommu core, it is really gross.
> 
> I would expect IDXD to keep track of the PASID's and mms it is using
> and do this kind of stuff itself.
> 
> And why is this using access_remote_vm anyhow? If you know you are in
> a kthread then kthread_use_mm() is probably better anyhow.
> 
> In any event we don't need a iommu_sva_find() function to wrapper
> xa_load for another function inside the same .c file.

Ok. I will maintain mm and find mm from PASID inside IDXD driver. And 
will implement accessing the remote mm inside IDXD driver although the 
implementation will have duplicate code as access_remote_vm().

Thanks.

-Fenghua