linux-kernel - RE: [PATCH v6 2/6] iommufd: Add IOMMU_HWPT

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Date:   Tue, 28 Nov 2023 08:03:35 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Nicolin Chen <nicolinc@...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>,
        "jgg@...dia.com" <jgg@...dia.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>,
        "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>,
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        "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>,
        "joao.m.martins@...cle.com" <joao.m.martins@...cle.com>,
        "Zeng, Xin" <xin.zeng@...el.com>,
        "Zhao, Yan Y" <yan.y.zhao@...el.com>
Subject: RE: [PATCH v6 2/6] iommufd: Add IOMMU_HWPT_INVALIDATE

> From: Nicolin Chen <nicolinc@...dia.com>
> Sent: Tuesday, November 28, 2023 3:53 AM
> 
> On Fri, Nov 24, 2023 at 02:36:29AM +0000, Tian, Kevin wrote:
> 
> > > > > > > >> + * @out_driver_error_code: Report a driver speicifc error code
> > > upon
> > > > > > > failure.
> > > > > > > >> + *                         It's optional, driver has a choice to fill it or
> > > > > > > >> + *                         not.
> > > > > > > >
> > > > > > > > Being optional how does the user tell whether the code is filled
> or
> > > not?
> > > > >
> > > > > Well, naming it "error_code" indicates zero means no error while
> > > > > non-zero means something? An error return from this ioctl could
> > > > > also tell the user space to look up for this driver error code,
> > > > > if it ever cares.
> > > >
> > > > probably over-thinking but I'm not sure whether zero is guaranteed to
> > > > mean no error in all implementations...
> > >
> > > Well, you are right. Usually HW conveniently raises a flag in a
> > > register to indicate something wrong, yet it is probably unsafe
> > > to say it definitely.
> > >
> >
> > this reminds me one open. What about an implementation having
> > a hierarchical error code layout e.g. one main error register with
> > each bit representing an error category then multiple error code
> > registers each for one error category? In this case probably
> > a single out_driver_error_code cannot carry that raw information.
> 
> Hmm, good point.
> 
> > Instead the iommu driver may need to define a customized error
> > code convention in uapi header which is converted from the
> > raw error information.
> >
> > From this angle should we simply say that the error code definition
> > must be included in the uapi header? If raw error information can
> > be carried by this field then this hw can simply say that the error
> > code format is same as the hw spec defines.
> >
> > With that explicit information then the viommu can easily tell
> > whether error code is filled or not based on its own convention.
> 
> That'd be to put this error_code field into the driver uAPI
> structure right?
> 
> I also thought about making this out_driver_error_code per HW.
> Yet, an error can be either per array or per entry/quest. The
> array-related error should be reported in the array structure
> that is a core uAPI, v.s. the per-HW entry structure. Though
> we could still report an array error in the entry structure
> at the first entry (or indexed by "array->entry_num")?
> 

why would there be an array error? array is just a software
entity containing actual HW invalidation cmds. If there is
any error with the array itself it should be reported via
ioctl errno.

Jason, how about your opinion? I didn't spot big issues
except this one. Hope it can make into 6.8.