Re: [PATCH v3 4/4] KVM: x86: AMD's IBPB is not equivalent to Intel's IBPB

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

On 8/23/24 15:51, Sean Christopherson wrote:
> On Fri, Aug 23, 2024, Tom Lendacky wrote:
>> On 8/23/24 13:53, Jim Mattson wrote:
>>> From Intel's documention [1], "CPUID.(EAX=07H,ECX=0):EDX[26]
>>> enumerates support for indirect branch restricted speculation (IBRS)
>>> and the indirect branch predictor barrier (IBPB)." Further, from [2],
>>> "Software that executed before the IBPB command cannot control the
>>> predicted targets of indirect branches (4) executed after the command
>>> on the same logical processor," where footnote 4 reads, "Note that
>>> indirect branches include near call indirect, near jump indirect and
>>> near return instructions. Because it includes near returns, it follows
>>> that **RSB entries created before an IBPB command cannot control the
>>> predicted targets of returns executed after the command on the same
>>> logical processor.**" [emphasis mine]
>>>
>>> On the other hand, AMD's IBPB "may not prevent return branch
>>> predictions from being specified by pre-IBPB branch targets" [3].
>>>
>>> However, some AMD processors have an "enhanced IBPB" [terminology
>>> mine] which does clear the return address predictor. This feature is
>>> enumerated by CPUID.80000008:EDX.IBPB_RET[bit 30] [4].
>>>
>>> Adjust the cross-vendor features enumerated by KVM_GET_SUPPORTED_CPUID
>>> accordingly.
>>>
>>> [1] https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/cpuid-enumeration-and-architectural-msrs.html
>>> [2] https://www.intel.com/content/www/us/en/developer/articles/technical/software-security-guidance/technical-documentation/speculative-execution-side-channel-mitigations.html#Footnotes
>>> [3] https://www.amd.com/en/resources/product-security/bulletin/amd-sb-1040.html
>>> [4] https://www.amd.com/content/dam/amd/en/documents/processor-tech-docs/programmer-references/24594.pdf
>>>
>>> Fixes: 0c54914d0c52 ("KVM: x86: use Intel speculation bugs and features as derived in generic x86 code")
>>> Suggested-by: Venkatesh Srinivas <venkateshs@chromium.org>
>>> Signed-off-by: Jim Mattson <jmattson@google.com>
>>> ---
>>>  arch/x86/kvm/cpuid.c | 6 +++++-
>>>  1 file changed, 5 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
>>> index ec7b2ca3b4d3..c8d7d928ffc7 100644
>>> --- a/arch/x86/kvm/cpuid.c
>>> +++ b/arch/x86/kvm/cpuid.c
>>> @@ -690,7 +690,9 @@ void kvm_set_cpu_caps(void)
>>>  	kvm_cpu_cap_set(X86_FEATURE_TSC_ADJUST);
>>>  	kvm_cpu_cap_set(X86_FEATURE_ARCH_CAPABILITIES);
>>>  
>>> -	if (boot_cpu_has(X86_FEATURE_IBPB) && boot_cpu_has(X86_FEATURE_IBRS))
>>> +	if (boot_cpu_has(X86_FEATURE_AMD_IBPB_RET) &&
>>> +	    boot_cpu_has(X86_FEATURE_AMD_IBPB) &&
>>> +	    boot_cpu_has(X86_FEATURE_AMD_IBRS))
>>>  		kvm_cpu_cap_set(X86_FEATURE_SPEC_CTRL);
>>>  	if (boot_cpu_has(X86_FEATURE_STIBP))
>>>  		kvm_cpu_cap_set(X86_FEATURE_INTEL_STIBP);
>>> @@ -759,6 +761,8 @@ void kvm_set_cpu_caps(void)
>>>  	 * arch/x86/kernel/cpu/bugs.c is kind enough to
>>>  	 * record that in cpufeatures so use them.
>>>  	 */
>>> +	if (boot_cpu_has(X86_FEATURE_SPEC_CTRL))
>>> +		kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB_RET);
>>
>> If SPEC_CTRL is set, then IBPB is set, so you can't have AMD_IBPB_RET
>> without AMD_IBPB, but it just looks odd seeing them set with separate
>> checks with no relationship dependency for AMD_IBPB_RET on AMD_IBPB.
>> That's just me, though, not worth a v4 unless others feel the same.
> 
> You thinking something like this (at the end, after the dust settles)?
> 
> 	if (WARN_ON_ONCE(kvm_cpu_cap_has(X86_FEATURE_AMD_IBPB_RET) &&
> 			 !kvm_cpu_cap_has(X86_FEATURE_AMD_IBPB)))
> 		kvm_cpu_cap_clear(X86_FEATURE_AMD_IBPB_RET);		

I was just thinking more along the lines of:

	if (boot_cpu_has(X86_FEATURE_IBPB)) {
		kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB);
		if (boot_cpu_has(X86_FEATURE_SPEC_CTRL))
			kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB_RET);
	}

Thanks,
Tom

>>
> 
>> Thanks,
>> Tom
>>
>>>  	if (boot_cpu_has(X86_FEATURE_IBPB))
>>>  		kvm_cpu_cap_set(X86_FEATURE_AMD_IBPB);
>>>  	if (boot_cpu_has(X86_FEATURE_IBRS))