ctfconvert problems with build on illumos based platform

ctfconvert problems with build on illumos based platform

[Igor Kozhukhov]

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Hello All,

I have problems with ctfconvert with build sources of xen-4.2 on illumos based platform:
https://www.illumos.org/issues/3205

could you please let me know - what the reason to have a variable in structure 'struct hvm_hw_cpu_xsave'  with zero size:
struct { char x[0]; } ymm;    /* YMM */
--
Best regards,
Igor Kozhukhov





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Re: ctfconvert problems with build on illumos based platform

[Andrew Cooper]

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On 14/01/14 20:38, Igor Kozhukhov wrote:
> Hello All,
>
> I have problems with ctfconvert with build sources of xen-4.2 on
> illumos based platform:
> https://www.illumos.org/issues/3205
>
> could you please let me know - what the reason to have a variable in
> structure 'struct hvm_hw_cpu_xsave'  with zero size:
> struct { char x[0]; } ymm;    /* YMM */
> --
> Best regards,
> Igor Kozhukhov
>
>

The structure is variable length depending on whether the VM has enabled
AVX support.

It is rather unfortunate that we have non-complient C used to specify
the ABI.

~Andrew

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Re: ctfconvert problems with build on illumos based platform

[Igor Kozhukhov]

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On Jan 15, 2014, at 12:46 AM, Andrew Cooper wrote:

> On 14/01/14 20:38, Igor Kozhukhov wrote:
>> Hello All,
>> 
>> I have problems with ctfconvert with build sources of xen-4.2 on illumos based platform:
>> https://www.illumos.org/issues/3205
>> 
>> could you please let me know - what the reason to have a variable in structure 'struct hvm_hw_cpu_xsave'
>>          with zero size:
>> struct { char x[0]; } ymm;    /* YMM */
>> --
>> Best regards,
>> Igor Kozhukhov
>> 
>> 
> 
> The structure is variable length depending on whether the VM has enabled AVX support.
> 
> It is rather unfortunate that we have non-complient C used to specify the ABI.

can we use here ?
struct { char x[1]; } ymm;

it will fix my problem.

-Igor


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Re: ctfconvert problems with build on illumos based platform

[Andrew Cooper]

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On 14/01/14 20:52, Igor Kozhukhov wrote:
>
> On Jan 15, 2014, at 12:46 AM, Andrew Cooper wrote:
>
>> On 14/01/14 20:38, Igor Kozhukhov wrote:
>>> Hello All,
>>>
>>> I have problems with ctfconvert with build sources of xen-4.2 on
>>> illumos based platform:
>>> https://www.illumos.org/issues/3205
>>>
>>> could you please let me know - what the reason to have a variable in
>>> structure 'struct hvm_hw_cpu_xsave'  with zero size:
>>> struct { char x[0]; } ymm;    /* YMM */
>>> --
>>> Best regards,
>>> Igor Kozhukhov
>>>
>>>
>>
>> The structure is variable length depending on whether the VM has
>> enabled AVX support.
>>
>> It is rather unfortunate that we have non-complient C used to specify
>> the ABI.
>
> can we use here ?
> struct { char x[1]; } ymm;
>
> it will fix my problem.
>
> -Igor
>

No, it wont.

The 'ymm' is either not present and has a size of 0 as far as save_area
is concerned, or is present, and has a size of 16 * 256bit registers.

The content of this structure is only relevant as far as
hvm_{save,load}_cpu_xsave_states() goes, which resorts to some
pointer-trickary to move the data.  Changing that char x[0] to char x[1]
will break the pointer trickary, and migration will suffer an ABI breakage.

~Andrew

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Re: ctfconvert problems with build on illumos based platform

[Ian Campbell]

On Tue, 2014-01-14 at 20:58 +0000, Andrew Cooper wrote:

> The content of this structure is only relevant as far as
> hvm_{save,load}_cpu_xsave_states() goes, which resorts to some
> pointer-trickary to move the data.  Changing that char x[0] to char
> x[1] will break the pointer trickary, and migration will suffer an ABI
> breakage.

The pointer-trickery will need updating at the same time then, that's
not rocket science and the use of a single char array is the normal
portable way to do these sorts of variable length suffixes to structs.

Ian.