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

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Date:   Fri, 4 Aug 2023 11:03:45 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     "Liu, Yi L" <yi.l.liu@...el.com>
Cc:     "Tian, Kevin" <kevin.tian@...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

On Fri, Aug 04, 2023 at 01:04:57PM +0000, Liu, Yi L wrote:
> > > Having the driver copy in a loop might be better
> > >
> > 
> > Can you elaborate?
> 
> I think Jason means the way in patch 09.

Yeah, you can't reuse the stack buffer for an array, so patch 9 copies
each element uniquely.

This is more calls to copy_to_user, which has some cost

But we avoid a memory allocation

Patch 9 should not abuse the user_data, cast it to the inv_info and
just put req on the stack:

	struct iommu_hwpt_vtd_s1_invalidate *inv_info = user_data;
	struct iommu_hwpt_vtd_s1_invalidate_desc req;

But I'm not sure about this entry_size logic, what happens if the
entry_size is larger than the kernel supports? I think it should
fail..

Jason