linux-kernel - Re: [PATCH 00/12] iommufd: Add nesting infrastructure

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Message-ID: <0193b5e3-68b5-16e8-b4e8-7fc5a3175a25@linux.intel.com>
Date:   Thu, 9 Mar 2023 22:02:21 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     Yi Liu <yi.l.liu@...el.com>, joro@...tes.org,
        alex.williamson@...hat.com, jgg@...dia.com, kevin.tian@...el.com,
        robin.murphy@....com
Cc:     baolu.lu@...ux.intel.com, cohuck@...hat.com, eric.auger@...hat.com,
        nicolinc@...dia.com, kvm@...r.kernel.org, mjrosato@...ux.ibm.com,
        chao.p.peng@...ux.intel.com, yi.y.sun@...ux.intel.com,
        peterx@...hat.com, jasowang@...hat.com,
        shameerali.kolothum.thodi@...wei.com, lulu@...hat.com,
        suravee.suthikulpanit@....com, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH 00/12] iommufd: Add nesting infrastructure

On 2023/3/9 16:08, Yi Liu wrote:
> Nested translation is a hardware feature that is supported by many modern
> IOMMU hardwares. It has two stages (stage-1, stage-2) address translation
> to get access to the physical address. stage-1 translation table is owned
> by userspace (e.g. by a guest OS), while stage-2 is owned by kernel. Changes
> to stage-1 translation table should be followed by an IOTLB invalidation.

The last sentence is not always true. If any entry of the stage-1
translation table is changed from non-present to present, the IOTLB has
no chance to cache it yet. Hence there's no need for invalidation.

Best regards,
baolu