On 31/12/2021 2:13 am, Sean Christopherson wrote:
On Fri, Dec 10, 2021, Like Xu wrote:
From: Like Xu <like.xu@xxxxxxxxxxxxxxx>
From: Like Xu <like.xu@xxxxxxxxxxxxxxx>
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
