Re: [PATCH v1 3/3] x86/bugs: Switch the selection of mitigation from CPU vendor to CPU features

[Tom Lendacky <thomas.lendacky@amd.com>]

On 6/11/2018 9:01 AM, Konrad Rzeszutek Wilk wrote:
> On Fri, Jun 08, 2018 at 04:30:15PM -0500, Tom Lendacky wrote:
>> On 6/1/2018 9:59 AM, Konrad Rzeszutek Wilk wrote:
>>> Both AMD and Intel can have SPEC CTRL MSR for SSBD.
>>>
>>> However AMD also has two more other ways of doing it - which
>>> are !SPEC_CTRL MSR ways.
>>>
>>> Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>>>
>>> ---
>>> Cc: Thomas Gleixner <tglx@linutronix.de>
>>> Cc: Ingo Molnar <mingo@redhat.com>
>>> Cc: "H. Peter Anvin" <hpa@zytor.com>
>>> Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
>>> Cc: Borislav Petkov <bp@suse.de>
>>> Cc: David Woodhouse <dwmw@amazon.co.uk>
>>> Cc: Kees Cook <keescook@chromium.org>
>>> Cc: KarimAllah Ahmed <karahmed@amazon.de>
>>> ---
>>>  arch/x86/kernel/cpu/bugs.c | 11 +++--------
>>>  1 file changed, 3 insertions(+), 8 deletions(-)
>>>
>>> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
>>> index 6bea81855cdd..cd0fda1fff6d 100644
>>> --- a/arch/x86/kernel/cpu/bugs.c
>>> +++ b/arch/x86/kernel/cpu/bugs.c
>>> @@ -532,17 +532,12 @@ static enum ssb_mitigation __init __ssb_select_mitigation(void)
>>>  		 * Intel uses the SPEC CTRL MSR Bit(2) for this, while AMD may
>>>  		 * use a completely different MSR and bit dependent on family.
>>>  		 */
>>> -		switch (boot_cpu_data.x86_vendor) {
>>> -		case X86_VENDOR_INTEL:
>>> -		case X86_VENDOR_AMD:
>>> -			if (!static_cpu_has(X86_FEATURE_MSR_SPEC_CTRL)) {
>>> -				x86_amd_ssb_disable();
>>> -				break;
>>> -			}
>>> +		if (!static_cpu_has(X86_FEATURE_MSR_SPEC_CTRL))
>>> +			x86_amd_ssb_disable();
>>> +		else {
>>
>> As I think about this more, I don't think we can do this for AMD.  The
>> X86_FEATURE_SSBD could be true because of the LS_CFG support and not the
>> AMD_SSBD CPUID bit.  But if the IBRS CPUID bit was set, we would now try
>> to use the SPEC_CTRL register for SSBD, which is not valid.
> 
> I was reading the AMD docs and while the SPEC CTRL provides IBRS my understanding
> (from reading between the lines) is that AMD would actually never implement this.
> 
> That is it would have the 'Enhanced IBRS' bit exposed if at all, but nothing else.
> 
> Granted this is tea-reading at its best so, ..
>>
>> I think you will have to keep the case statements and explicitly check for
>> X86_FEATURE_AMD_SSBD before using SPEC_CTRL.
> 
> .. we could or alternatively add an extra check for X86_FEATURE_AMD_SSBD ?

Whichever you feel is best, so long as we only use SPEC_CTRL for SSBD on
AMD when X86_FEATURE_AMD_SSBD is present.

Thanks,
Tom

> 
> 
> I think Thomas already sent this out but it should be no problems to
> add a fix as there is no hardware with this so it isn't like we are
> breaking anything :-)
>