linux-kernel - RE: [PATCH v4 04/12] iommu/vt-d: Add 256-bit invalidation descriptor support

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Message-ID: <A2975661238FB949B60364EF0F2C257439D60599@SHSMSX104.ccr.corp.intel.com>
Date:   Thu, 8 Nov 2018 05:48:37 +0000
From:   "Liu, Yi L" <yi.l.liu@...el.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>
CC:     "Raj, Ashok" <ashok.raj@...el.com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "Tian, Kevin" <kevin.tian@...el.com>,
        "Sun, Yi Y" <yi.y.sun@...el.com>,
        "peterx@...hat.com" <peterx@...hat.com>,
        Jean-Philippe Brucker <jean-philippe.brucker@....com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>
Subject: RE: [PATCH v4 04/12] iommu/vt-d: Add 256-bit invalidation
 descriptor support

> From: Liu, Yi L
> Sent: Thursday, November 8, 2018 1:45 PM
> > >>>> +			memcpy(desc, qi->desc + (wait_index << shift),
> > >>>
> > >>> Would "memcpy(desc, (unsigned long long) (qi->desc +  (wait_index
> > >>> << shift)," be more safe?
> > >>
> > >> Can that be compiled? memcpy() requires a "const void *" for the
> > >> second
> > parameter.
> > >> By the way, why it's safer with this casting?
> > >
> > > This is just an example. My point is the possibility that "qi->desc
> > > + (wait_index <<
> > shift)"
> > > would be treated as "qi->desc plus (wait_index <<
> > > shift)*sizeof(*qi->desc)". Is it possible for kernel build?
> >
> > qi->desc is of type of "void *".
> 
> no, I don’t think so... Refer to the code below. Even it has no correctness issue her,
> It's not due to qi->desc is "void *" type...
> 
> struct qi_desc {
> -	u64 low, high;
> +	u64 qw0;
> +	u64 qw1;
> +	u64 qw2;
> +	u64 qw3;
> };

Oops, just see you modified it to be "void *" in this patch. Ok, then this is fair enough.

Thanks,
Yi Liu