Re: [Xen-devel] [PATCH v5 for-4.9 1/4] hvm/dmop: Box dmop_bufs rather than passing two parameters around

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 10/04/17 10:40, Paul Durrant wrote:
>>
>>> Why is this a good thing? Passing two parameters around allowed for them
>> to be in registers. I preferred the code as it was before.
>>
>> a) It will always be inlined, so registers aren't relevant.
> Why? I see nothing forcing the compiler to make it so.

Fine.  Let me rephrase as "GCC does inline it".

>
>>  Even if
>> they were, all values are available directly with the pointer as a base,
>> so there is no reduction in expressiveness.  (i.e. the previous code
>> only increases register pressure).
>> b) passing multiple parameters like that is a recipe for mistakes, and
>> in this case, mistakes mean security vulnerabilities.
> Given the locality of the code I don't buy that as an argument unless you're 
> going to assert that passing more than one parameter is always wrong.

Passing more than one parameter is of course fine.

Requiring the caller to pass two parameters which strictly must be in
sync for security reasons is not fine.

~Andrew

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