On Tue, Dec 6, 2016 at 1:40 AM, Borislav Petkov <b...@alien8.de> wrote:
> On Mon, Dec 05, 2016 at 05:01:14PM -0800, Andy Lutomirski wrote:
>> The old code didn't work at all because it adjusted the current caps
>> instead of the forced caps. Anything it did would be undone later
>> during cpu identification. Fix that and, while we're at it, improve
>> the logging and don't bother running it if CPUID is available.
>>
>> Signed-off-by: Andy Lutomirski <l...@kernel.org>
>> ---
>> arch/x86/kernel/fpu/init.c | 28 ++++++++++++++++------------
>> 1 file changed, 16 insertions(+), 12 deletions(-)
>>
>> diff --git a/arch/x86/kernel/fpu/init.c b/arch/x86/kernel/fpu/init.c
>> index 60dece392b3a..75e1bf3b0319 100644
>> --- a/arch/x86/kernel/fpu/init.c
>> +++ b/arch/x86/kernel/fpu/init.c
>> @@ -50,29 +50,33 @@ void fpu__init_cpu(void)
>>
>> /*
>> * The earliest FPU detection code.
>> - *
>> - * Set the X86_FEATURE_FPU CPU-capability bit based on
>> - * trying to execute an actual sequence of FPU instructions:
>> */
>> static void fpu__init_system_early_generic(struct cpuinfo_x86 *c)
>> {
>> - unsigned long cr0;
>> - u16 fsw, fcw;
>> + if (!boot_cpu_has(X86_FEATURE_CPUID) &&
>> + !test_bit(X86_FEATURE_FPU, (unsigned long *)cpu_caps_cleared)) {
>
> Flip test and save an indentation level.
How? There's that bit at the bottom to worry about.
>
>> + /*
>> + * Set the X86_FEATURE_FPU CPU-capability bit based on
>> + * trying to execute an actual sequence of FPU instructions:
>> + */
>>
>> - fsw = fcw = 0xffff;
>> + unsigned long cr0;
>> + u16 fsw, fcw;
>>
>> - cr0 = read_cr0();
>> - cr0 &= ~(X86_CR0_TS | X86_CR0_EM);
>> - write_cr0(cr0);
>> + fsw = fcw = 0xffff;
>> +
>> + cr0 = read_cr0();
>> + cr0 &= ~(X86_CR0_TS | X86_CR0_EM);
>> + write_cr0(cr0);
>>
>> - if (!test_bit(X86_FEATURE_FPU, (unsigned long *)cpu_caps_cleared)) {
>> asm volatile("fninit ; fnstsw %0 ; fnstcw %1"
>> : "+m" (fsw), "+m" (fcw));
>> + pr_info("x86/fpu: FSW=0x%04hx FCW=0x%04hx\n", fsw, fcw);
>
> Why do we want those in dmesg? Maybe KERN_DEBUG?
For debugging, since this code appears busted in every version of
Linux I looked at. It's certainly been busted for quite a few years.
And this line won't print on any CPU with CPUID, so it's not going to
cause widespread spam. I kind of line the idea of being able to ask
users of these ancient CPUs to just send in their logs.
--Andy
