Re: [PATCH 1/7] x86/feature: Detect the x86 feature to control Speculation

Tim Chen
2018-01-05T17:07:05Z

On 01/05/2018 07:14 AM, Tom Lendacky wrote:
> On 1/5/2018 5:14 AM, David Woodhouse wrote:
>> On Thu, 2018-01-04 at 09:56 -0800, Tim Chen wrote:
>>> cpuid ax=0x7, return rdx bit 26 to indicate presence of this feature
>>> IA32_SPEC_CTRL (0x48) and IA32_PRED_CMD (0x49)
>>> IA32_SPEC_CTRL, bit0 – Indirect Branch Restricted Speculation (IBRS)
>>> IA32_PRED_CMD, bit0 – Indirect Branch Prediction Barrier (IBPB)
>>
>> In a previous iteration of these patches, hadn't you already merged
>> support for the AMD variant? Where'd that go?
>
> It looks like this series is strictly IBRS related. Even though some of
> the IBPB definitions are in here, there isn't support later for indicating
> IBPB is in use or any places where IBPB would be used. I was assuming that
> there would be another series for IBPB support. Is that the plan?

IBPB is meant to be a separate series. We want to get retpoline
and IBRS out first.

>
> AMD is covered by the IBRS support as is, but we also have support for
> the IBPB feature alone, identified by a different CPUID bit. I was
> waiting for the IBPB series to see if I needed to submit anything or
> whether the patches were included.
>

Tim