linux-kernel - Re: [PATCH v2 02/12] iommu/vt-d: Manage scalalble mode PASID tables

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Message-ID: <7e5faa39-376e-2c77-3891-9aac9092524a@linux.intel.com>
Date:   Fri, 7 Sep 2018 09:57:38 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Jacob Pan <jacob.jun.pan@...el.com>
Cc:     baolu.lu@...ux.intel.com, "Tian, Kevin" <kevin.tian@...el.com>,
        Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        "Liu, Yi L" <yi.l.liu@...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 v2 02/12] iommu/vt-d: Manage scalalble mode PASID tables

Hi,

On 09/07/2018 07:43 AM, Jacob Pan wrote:
> On Thu, 6 Sep 2018 10:46:03 +0800
> Lu Baolu <baolu.lu@...ux.intel.com> wrote:
> 
>>>> @@ -224,7 +271,14 @@ struct pasid_entry
>>>> *intel_pasid_get_entry(struct device *dev, int pasid)
>>>>     */
>>>>    static inline void pasid_clear_entry(struct pasid_entry *pe)
>>>>    {
>>>> -	WRITE_ONCE(pe->val, 0);
>>>> +	WRITE_ONCE(pe->val[0], 0);
>>>> +	WRITE_ONCE(pe->val[1], 0);
>>>> +	WRITE_ONCE(pe->val[2], 0);
>>>> +	WRITE_ONCE(pe->val[3], 0);
>>>> +	WRITE_ONCE(pe->val[4], 0);
>>>> +	WRITE_ONCE(pe->val[5], 0);
>>>> +	WRITE_ONCE(pe->val[6], 0);
>>>> +	WRITE_ONCE(pe->val[7], 0);
>>>
>>> memset?
>>
>> The order is important here. Otherwise, the PRESENT bit of this pasid
>> entry might still set while other fields contains invalid values.
> 
> WRITE_ONCE/READ_ONCE will switch to __builtin_memcpy() in if the size
> exceeds word size, ie. 64bit in this case. I don;t think compiler will
> reorder built-in function. Beside, we only need to clear present and
> FDP bit, right?

Clear present and FDP bit is enough for hardare. But from software point
of view, it's better to clear all bits with 0.

Best regards,
Lu Baolu