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Re: [Xen-devel] [PATCH v2] libx86: Work around GCC bug with ebx output constrants
- To: Jan Beulich <JBeulich@xxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Mon, 19 Nov 2018 15:19:42 +0000
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- Cc: Xen-devel <xen-devel@xxxxxxxxxxxxx>, Wei Liu <wei.liu2@xxxxxxxxxx>, Roger Pau Monne <roger.pau@xxxxxxxxxx>
- Delivery-date: Mon, 19 Nov 2018 15:20:05 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Openpgp: preference=signencrypt
On 19/11/2018 15:14, Jan Beulich wrote:
>>>> On 19.11.18 at 15:45, <andrew.cooper3@xxxxxxxxxx> wrote:
>> Versions of GCC before 5 can't compile cpuid.c, and fail with the rather
>> cryptic:
>>
>> In file included from lib/x86/cpuid.c:3:0:
>> lib/x86/cpuid.c: In function ‘x86_cpuid_policy_fill_native’:
>> include/xen/lib/x86/cpuid.h:25:5: error: inconsistent operand constraints
>> in an ‘asm’
>> asm ( "cpuid"
>> ^
>>
>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>> ---
>> CC: Jan Beulich <JBeulich@xxxxxxxx>
>> CC: Wei Liu <wei.liu2@xxxxxxxxxx>
>> CC: Roger Pau Monné <roger.pau@xxxxxxxxxx>
>>
>> v2:
>> * GCC 5 is fine. Its cpuid instrinct has none of the PIC workarounds thant
>> 4.9 have.
>> * Fix 64bit builds with larger models.
> It is rather odd that 64-bit is also affected - the error gets raised
> even when there's no use of %rbx for GOT accesses. By when
> they need a callee-saved register, they indeed use %rbx instead
> to the ABI-suggested %r15.
>
>> --- a/xen/include/xen/lib/x86/cpuid.h
>> +++ b/xen/include/xen/lib/x86/cpuid.h
>> @@ -20,21 +20,50 @@ struct cpuid_leaf
>> uint32_t a, b, c, d;
>> };
>>
>> +/*
>> + * Versions of GCC before 5 are unable to cope with %rBX output constraints
>> + * when compiling Position Independent Code, and produce a rather cryptic
>> + * error:
>> + * error: inconsistent operand constraints in an ‘asm’
>> + *
>> + * To work around the issue, use a separate register to hold the the %rBX
>> + * output, and xchg twice to leave %rBX preserved around the asm()
>> statement.
>> + */
>> +#if defined(__PIC__) && __GNUC__ < 5 && !defined(__clang__) &&
>> defined(__i386__)
>> +# define XCHG_BX "xchg %%ebx, %[bx];"
>> +# define BX_CON [bx] "=&r"
>> +#elif defined(__PIC__) && __GNUC__ < 5 && !defined(__clang__) && \
>> + defined(__x86_64__) && (defined(__code_model_medium__) || \
>> + defined(__code_model_large__))
>> +# define XCHG_BX "xchg %%rbx, %q[bx];"
>> +# define BX_CON [bx] "=&r"
>> +#else
>> +# define XCHG_BX ""
>> +# define BX_CON "=&b"
> The & is unnecessary here I think. Preferably with it dropped
> Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
That was to match the "=&d" and friends. I'd prefer to be consistent
(one way or the other).
~Andrew
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