On 8/17/2023 5:33 AM, Sean Christopherson wrote:
On Wed, Aug 16, 2023, Kai Huang wrote:
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -7783,6 +7783,9 @@ static void vmx_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
vmx->msr_ia32_feature_control_valid_bits &=
~FEAT_CTL_SGX_LC_ENABLED;
+ if (boot_cpu_has(X86_FEATURE_LAM))
+ kvm_governed_feature_check_and_set(vcpu, X86_FEATURE_LAM);
+
If you want to use boot_cpu_has(), it's better to be done at your last patch to
only set the cap bit when boot_cpu_has() is true, I suppose.
Yes, but new version of kvm_governed_feature_check_and_set() of
KVM-governed feature framework will check against kvm_cpu_cap_has() as well.
I will remove the if statement and call
kvm_governed_feature_check_and_set() directly.
https://lore.kernel.org/kvm/20230815203653.519297-2-seanjc@xxxxxxxxxx/
I mean kvm_cpu_cap_has() checks against the host CPUID directly while here you
are using boot_cpu_has(). They are not the same.
If LAM should be only supported when boot_cpu_has() is true then it seems you
can just only set the LAM cap bit when boot_cpu_has() is true. As you also
mentioned above the kvm_governed_feature_check_and_set() here internally does
kvm_cpu_cap_has().
That's covered by the last patch:
diff --git a/arch/x86/kvm/cpuid.c b/arch/x86/kvm/cpuid.c
index e961e9a05847..06061c11d74d 100644
--- a/arch/x86/kvm/cpuid.c
+++ b/arch/x86/kvm/cpuid.c
@@ -677,7 +677,7 @@ void kvm_set_cpu_caps(void)
kvm_cpu_cap_mask(CPUID_7_1_EAX,
F(AVX_VNNI) | F(AVX512_BF16) | F(CMPCCXADD) |
F(FZRM) | F(FSRS) | F(FSRC) |
- F(AMX_FP16) | F(AVX_IFMA)
+ F(AMX_FP16) | F(AVX_IFMA) | F(LAM)
);
kvm_cpu_cap_init_kvm_defined(CPUID_7_1_EDX,
Which highlights a problem with activating a goverened feature before said feature
is actually supported by KVM: it's all kinds of confusing.
It'll generate a more churn in git history, but I think we should first enable
LAM without a goverened feature, and then activate a goverened feature later on.
Using a goverened feature is purely an optimization, i.e. the series needs to be
function without using a governed feature.
OK, then how about the second option which has been listed in your v9
patch series discussion.
https://lore.kernel.org/kvm/20230606091842.13123-1-binbin.wu@xxxxxxxxxxxxxxx/T/#m16ee5cec4a46954f985cb6afedb5f5a3435373a1
Temporarily add a bool can_use_lam in kvm_vcpu_arch and use the bool
"can_use_lam" instead of guest_can_use(vcpu, X86_FEATURE_LAM).
and then put the patch of adopting "KVM-governed feature framework" to
the last.
That should yield an easier-to-review series on all fronts: the initial supports
won't have any more hidden dependencies than absolutely necessary, switching to
a goverened feature should be a very mechanical conversion (if it's not, that's
a red flag), and last but not least, it makes it super easy to make a judgment
call as to whether using a governed feature flag is justified, because all of the
users will be in scope.
TL;DR: Do the whole goverened feature thing dead last.
