Re: [PATCH 1/2] xen-detect: fix strict-aliasing compilation warning.

[Andrew Cooper <andrew.cooper3@citrix.com>]

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On 01/12/14 18:45, John Haxby wrote:
>
>> On 1 Dec 2014, at 17:15, Andrew Cooper <andrew.cooper3@citrix.com
>> <mailto:andrew.cooper3@citrix.com>> wrote:
>>
>> On 01/12/14 14:37, John Haxby wrote:
>>> With gcc 4.8.3, compiling xen-detect gives a compilation warning if
>>> you're optimising:
>>>
>>> $ cc -Wall -Os xen-detect.c
>>> xen-detect.c: In function ‘check_for_xen’:
>>> xen-detect.c:65:9: warning: dereferencing type-punned pointer will break
>>> strict-aliasing rules [-Wstrict-aliasing]
>>>         *(uint32_t *)(signature + 0) = regs[1];
>>>         ^
>>>
>>> Signed-off-by: John Haxby <john.haxby@oracle.com
>>> <mailto:john.haxby@oracle.com>>
>>
>> Why are you compiling without the CFLAGS from the Xen build system?
>>
>> We explicitly disable strict alias optimisations, because optimisations
>> based upon the aliasing rules in C is mad.  Even when you eliminate all
>> the warnings, there are still subtle bugs because the compiler is free
>> to assume a lot more than a programmer would typically deem reasonable.
>
>
> I wasn’t building the whole system, I just wanted xen-detect so I
> pulled it out and compiled it; I usually use "-Wall -Os” because the
> combination finds problems I might otherwise overlook.   The patch
> also removes three lines of code :) but you can take it or leave it as
> you choose.   The other patch — reversing the code of pv and hvm
> checking — was the real problem.
>
> jch

I feel it would be neater to fix this by using the
XEN_CPUID_SIGNATURE_E{B,C,D}X constants from the API.  This fixes the
strict aliasing, and does away with the string handling completely.

~Andrew

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