Re: [PATCH v11 01/17] perf/x86/intel: Add EPT-Friendly PEBS for Ice Lake Server

From: Like Xu <like.xu.linux@gmail.com>
To: Sean Christopherson <seanjc@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>,
	Jim Mattson <jmattson@google.com>,
	Wanpeng Li <wanpengli@tencent.com>,
	Vitaly Kuznetsov <vkuznets@redhat.com>,
	Joerg Roedel <joro@8bytes.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Like Xu <likexu@tencent.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v11 01/17] perf/x86/intel: Add EPT-Friendly PEBS for Ice Lake Server
Date: Fri, 31 Dec 2021 12:00:37 +0800	[thread overview]
Message-ID: <69ad949e-4788-0f93-46cb-6af6f79a9f24@gmail.com> (raw)
In-Reply-To: <Yc321e9o16luwFK+@google.com>

On 31/12/2021 2:13 am, Sean Christopherson wrote:
> On Fri, Dec 10, 2021, Like Xu wrote:
>> From: Like Xu <like.xu@linux.intel.com>
>>
>> From: Like Xu <like.xu@linux.intel.com>
> 
> Did one of these get handcoded?

Uh, now I have found the use of "--from=<ident>".

> 
>> The new hardware facility supporting guest PEBS is only available on
>> Intel Ice Lake Server platforms for now. KVM will check this field
>> through perf_get_x86_pmu_capability() instead of hard coding the cpu
>> models in the KVM code. If it is supported, the guest PEBS capability
>> will be exposed to the guest.
> 
> So what exactly is this new feature?  I've speed read the cover letter and a few
> changelogs and didn't find anything that actually explained when this feature does.
> 

Please check Intel SDM Vol3 18.9.5 for this "EPT-Friendly PEBS" feature.

I assume when an unfamiliar feature appears in the patch SUBJECT,
the reviewer may search for the exact name in the specification.

> Based on the shortlog, I assume the feature handles translating linear addresses
> via EPT?  If that's correct, then x86_pmu.pebs_vmx should be named something like
> x86_pmu.pebs_ept.

"Translating linear addresses via EPT" is only part of the hardware implementation,
and we may apply the new name if there are no other objections.

> 
> That also raises the question of what will happen if EPT is disabled.  Presumably
> things will Just Work since no additional translation is needed, but if that's the
> case then arguably vmx_pebs_supported() should be:
> 
> 	return boot_cpu_has(X86_FEATURE_PEBS) &&
> 	       (!tdp_enabled || kvm_pmu_cap.pebs_vmx);

Yes, a similar fix is already on my private tree, and thank you for pointing it out!

> 
> I'm guessing no one actually cares about supporting PEBS on older CPUs using shadow
> paging, but the changelog should at least call out that PEBS is allowed if and only
> if "pebs_vmx" is supported for simplicity, even though it would actually work if EPT
> is disabled.  And if for some reason it _doesn't_ work when EPT is disabled, then
> vmx_pebs_supported() and friends need to actually check tdp_enabled.

Yes, the guest PEBS only works when EPT is enabled on the newer modern CPUs.

> 
> Regardless, this changelog really, really needs an explanation of the feature.

Thank you for picking up, I will update the changelog for this commit.

Please let me know if you have any more obstacles or niggles to review this 
patch set.

Thanks,
Like Xu