linux-kernel - Re: [PATCH 1/1] x86/iommu: use bit structures for context

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Date:	Fri, 24 May 2013 08:43:04 +0800
From:	ZhenHua <zhen-hual@...com>
To:	Joe Perches <joe@...ches.com>
CC:	David Woodhouse <dwmw2@...radead.org>,
	iommu@...ts.linux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] x86/iommu: use bit structures for context_entry

Use lo and hi for clear, may run faster than memset.

But it is not a big problem.  Never mind.

Thanks
ZhenHua

On 05/24/2013 08:36 AM, Joe Perches wrote:
> On Fri, 2013-05-24 at 08:22 +0800, Li, Zhen-Hua wrote:
>> There is a structure named context_entry used by intel iommu, and there
>> are some bit operations on it. Use bit structure may make these operations
>> easy.
> []
>> diff --git a/drivers/iommu/intel-iommu.c b/drivers/iommu/intel-iommu.c
> []
>> @@ -221,55 +221,28 @@ get_context_addr_from_root(struct root_entry *root)
>>    * 8-23: domain id
>>    */
>>   struct context_entry {
>> -	u64 lo;
>> -	u64 hi;
>> +	union {
>> +		struct {
>> +			u64	present:1,
> why use struct and union at all?
>
> Why not just:
>
> struct context_entry {
> 	u64	present:1,
> 		fpd:1,
> 		translation_type:2,
> 		reserved_l:8,
> 		asr:52;
> 	u64	aw:3,
> 		avail:4,
> 		reserved_h1:1,
> 		did:16,
> 		reserved_h2:40;
> };
>
>> @@ -743,7 +716,8 @@ static void clear_context_table(struct intel_iommu *iommu, u8 bus, u8 devfn)
> []
>>   	if (context) {
>> -		context_clear_entry(&context[devfn]);
>> +		context[devfn].lo = 0;
>> +		context[devfn].hi = 0;
> 		memset(&context[devfn], 0, sizeof(...))
>
>

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