Re: [tip:x86/pti] x86/speculation: Use IBRS if available before calling into firmware

Tim Chen
2018-02-16T23:46:58Z

On 02/16/2018 11:16 AM, David Woodhouse wrote:
> On Fri, 2018-02-16 at 10:44 -0800, Tim Chen wrote:
>>
>> I encountered hang on a machine but not others when using the above
>> macro. It is probably an alignment thing with ALTERNATIVE as the
>> problem went
>> away after I made the change below:
>>
>> Tim
>>
>> diff --git a/arch/x86/include/asm/nospec-branch.h
>> b/arch/x86/include/asm/nospec-branch.h
>> index 8f2ff74..0f65bd2 100644
>> --- a/arch/x86/include/asm/nospec-branch.h
>> +++ b/arch/x86/include/asm/nospec-branch.h
>> @@ -148,6 +148,7 @@ extern char __indirect_thunk_end[];
>>
>> #define alternative_msr_write(_msr, _val, _feature) \
>> asm volatile(ALTERNATIVE("", \
>> + ".align 16\n\t" \
>> "movl %[msr], %%ecx\n\t" \
>> "movl %[val], %%eax\n\t" \
>> "movl $0, %%edx\n\t" \
>
> That's weird. Note that .align in an altinstr section isn't actually
> going to do what you'd expect; the oldinstr and altinstr sections
> aren't necessarily aligned the same, so however many NOPs it inserts
> into the alternative, might be deliberately *misaligning* it in the
> code that actually gets executed.
>
> Are you sure you're not running a kernel where the alternatives code
> would turn that alternative which *starts* with a NOP, into *all* NOPs?
>

I rebuild the kernel again without the align. I'm no longer
seeing the issue again on that machine that had an issue earlier.
So let's ignore this for now as I can't reproduce the problem.

It should be other issues causing the hang I saw earlier.

Thanks.

Tim