linux-kernel - RE: [PATCH v4 07/12] iommufd: Add data structure for Intel VT-d stage-1 cache invalidation

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Date:   Thu, 3 Aug 2023 00:38:53 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Jason Gunthorpe <jgg@...dia.com>
CC:     "Liu, Yi L" <yi.l.liu@...el.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "alex.williamson@...hat.com" <alex.williamson@...hat.com>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "baolu.lu@...ux.intel.com" <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 v4 07/12] iommufd: Add data structure for Intel VT-d
 stage-1 cache invalidation

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Wednesday, August 2, 2023 9:48 PM
> 
> On Wed, Aug 02, 2023 at 07:41:05AM +0000, Tian, Kevin wrote:
> > > +/**
> > > + * struct iommu_hwpt_vtd_s1_invalidate - Intel VT-d cache invalidation
> > > + *                                       (IOMMU_HWPT_TYPE_VTD_S1)
> > > + * @flags: Must be 0
> > > + * @entry_size: Size in bytes of each cache invalidation request
> > > + * @entry_nr_uptr: User pointer to the number of invalidation requests.
> > > + *                 Kernel reads it to get the number of requests and
> > > + *                 updates the buffer with the number of requests that
> > > + *                 have been processed successfully. This pointer must
> > > + *                 point to a __u32 type of memory location.
> > > + * @inv_data_uptr: Pointer to the cache invalidation requests
> > > + *
> > > + * The Intel VT-d specific invalidation data for a set of cache invalidation
> > > + * requests. Kernel loops the requests one-by-one and stops when
> failure
> > > + * is encountered. The number of handled requests is reported to user
> by
> > > + * writing the buffer pointed by @entry_nr_uptr.
> > > + */
> > > +struct iommu_hwpt_vtd_s1_invalidate {
> > > +	__u32 flags;
> > > +	__u32 entry_size;
> > > +	__aligned_u64 entry_nr_uptr;
> > > +	__aligned_u64 inv_data_uptr;
> > > +};
> > > +
> >
> > I wonder whether this array can be defined directly in the common
> > struct iommu_hwpt_invalidate so there is no need for underlying
> > iommu driver to further deal with user buffers, including various
> > minsz/backward compat. check.
> 
> You want to have an array and another chunk of data?
> 
> What is the array for? To do batching?

yes, it's for batching

> 
> It means we have to allocate memory on this path, that doesn't seem
> like the right direction for a performance improvement..

It reuses the ucmd_buffer to avoid memory allocation:

@@ -485,6 +485,12 @@ union ucmd_buffer {
 #ifdef CONFIG_IOMMUFD_TEST
 	struct iommu_test_cmd test;
 #endif
+	/*
+	 * hwpt_type specific structure used in the cache invalidation
+	 * path.
+	 */
+	struct iommu_hwpt_vtd_s1_invalidate vtd;
+	struct iommu_hwpt_vtd_s1_invalidate_desc req_vtd;
 };

I don't quite like this way.

> 
> Having the driver copy in a loop might be better
> 

Can you elaborate?