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:

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