linux-kernel - Re: [PATCH v3 07/10] iommu/vt-d: Add iotlb flush for nested domain

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Message-ID: <097a2abf-e817-99ca-1f31-dbd439aaade8@linux.intel.com>
Date:   Thu, 8 Jun 2023 16:07:33 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     "Liu, Yi L" <yi.l.liu@...el.com>,
        "Tian, Kevin" <kevin.tian@...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:     baolu.lu@...ux.intel.com, "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 v3 07/10] iommu/vt-d: Add iotlb flush for nested domain

On 2023/6/8 15:14, Liu, Yi L wrote:
>>> + *                                              stage-1 page table cache
>>> + *                                              invalidation
>>> + * @IOMMU_VTD_QI_FLAGS_LEAF: The LEAF flag indicates whether only the
>>> + *                           leaf PTE caching needs to be invalidated
>>> + *                           and other paging structure caches can be
>>> + *                           preserved.
>>> + */
>> what about "Drain Reads" and "Drain Writes"? Is the user allowed/required
>> to provide those hints?
> All other comments got. For these two hints, the two flags are from the IOTLB
> Invalidation descriptor. Per below description, the hardware that supports nested
> should support drain and does not require software to ask for it. So it appears no
> need to define them in uapi.
> 
> "Hardware implementation with Major Version 2 or higher (VER_REG),
> always performs required drain without software explicitly requesting
> a drain in IOTLB invalidation. This field is deprecated and hardware
> will always report it as 1 to maintain backward compatibility with
> software"

Make sense. Perhaps we can also remove below code in
__iommu_flush_iotlb():

         /* Note: set drain read/write */
#if 0
         /*
          * This is probably to be super secure.. Looks like we can
          * ignore it without any impact.
          */
         if (cap_read_drain(iommu->cap))
                 val |= DMA_TLB_READ_DRAIN;
#endif

Best regards,
baolu