linux-kernel - RE: [PATCH v4 09/12] iommu/vt-d: Add iotlb flush for nested domain

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Date:   Thu, 3 Aug 2023 04:13:11 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Baolu Lu <baolu.lu@...ux.intel.com>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "jgg@...dia.com" <jgg@...dia.com>,
        "robin.murphy@....com" <robin.murphy@....com>
CC:     "cohuck@...hat.com" <cohuck@...hat.com>,
        "eric.auger@...hat.com" <eric.auger@...hat.com>,
        "nicolinc@...dia.com" <nicolinc@...dia.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "mjrosato@...ux.ibm.com" <mjrosato@...ux.ibm.com>,
        "chao.p.peng@...ux.intel.com" <chao.p.peng@...ux.intel.com>,
        "yi.y.sun@...ux.intel.com" <yi.y.sun@...ux.intel.com>,
        "peterx@...hat.com" <peterx@...hat.com>,
        "jasowang@...hat.com" <jasowang@...hat.com>,
        "shameerali.kolothum.thodi@...wei.com" 
        <shameerali.kolothum.thodi@...wei.com>,
        "lulu@...hat.com" <lulu@...hat.com>,
        "suravee.suthikulpanit@....com" <suravee.suthikulpanit@....com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "Duan, Zhenzhong" <zhenzhong.duan@...el.com>
Subject: RE: [PATCH v4 09/12] iommu/vt-d: Add iotlb flush for nested domain

> From: Baolu Lu <baolu.lu@...ux.intel.com>
> Sent: Thursday, August 3, 2023 12:06 PM
> 
> On 2023/8/3 12:00, Tian, Kevin wrote:
> >> From: Baolu Lu <baolu.lu@...ux.intel.com>
> >> Sent: Thursday, August 3, 2023 11:25 AM
> >>
> >> On 2023/8/2 15:46, Tian, Kevin wrote:
> >>>> From: Liu, Yi L <yi.l.liu@...el.com>
> >>>> Sent: Monday, July 24, 2023 7:14 PM
> >>>>
> >>>> +
> >>>> +		spin_lock_irqsave(&dmar_domain->lock, flags);
> >>>> +		list_for_each_entry(info, &dmar_domain->devices, link)
> >>>> +			intel_nested_invalidate(info->dev, dmar_domain,
> >>>> +						req->addr, req->npages);
> >>>> +		spin_unlock_irqrestore(&dmar_domain->lock, flags);
> >>>
> >>> Disabling interrupt while invalidating iotlb is certainly unacceptable.
> >>>
> >>> Actually there is no need to walk devices. Under dmar_domain there
> >>> is already a list of attached iommu's.
> >>
> >> Walking device is only necessary when invalidating device TLB. For iotlb
> >> invalidation, it only needs to know the iommu's.
> >>
> >
> > even for device tlb we may think whether there is any better way
> > to avoid disabling interrupt. It's a slow path, especially in a guest.
> 
> I ever tried this. But some device drivers call iommu_unmap() in the
> interrupt critical path. :-( So we have a long way to go.
> 

emmm... this path only comes from iommufd and the domain is
user-managed. There won't be kernel drivers to call iommu_unmap()
on such domain.