Re: [Xen-devel] [PATCH 1/3] x86/emul: Introduce build time assertions for struct segment_register

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 03/07/17 13:29, Jan Beulich wrote:
>>>> On 30.06.17 at 17:04, <andrew.cooper3@xxxxxxxxxx> wrote:
>> This structure is shared with hardware in the AMD VMCB.
> Indeed, but do we really depend on that in the emulator code?
>
>> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
>> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
>> @@ -7899,6 +7899,12 @@ static void __init __maybe_unused 
>> build_assertions(void)
>>      BUILD_BUG_ON(X86_EVENTTYPE_SW_INTERRUPT != 4);
>>      BUILD_BUG_ON(X86_EVENTTYPE_PRI_SW_EXCEPTION != 5);
>>      BUILD_BUG_ON(X86_EVENTTYPE_SW_EXCEPTION != 6);
>> +
>> +    /* Check struct segment_register against the VMCB segment layout. */
>> +    BUILD_BUG_ON(sizeof(struct segment_register) != 16);
>> +    BUILD_BUG_ON(offsetof(struct segment_register, attr) != 2);
> I.e. for these two I don't think I can see any dependency, and ...
>
>> +    BUILD_BUG_ON(offsetof(struct segment_register, limit) != 4);
>> +    BUILD_BUG_ON(offsetof(struct segment_register, base) != 8);
> ... for these two I think all we require is >=. Otoh, if these were put
> in SVM code, then I could see the point of them being the way they
> are.

These are indeed only for the SVM code.  See the impending 4/3 patch
which I'm about to post.

>  I'd then even raise the question whether we wouldn't also want
> offsetof() checks.

I don't understand; these are offsetof checks.

~Andrew

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