Hi Fuad,

On Tue, Feb 1, 2022 at 6:14 AM Fuad Tabba [off-list ref] wrote:

Hi Reiji,

...

quoted

quoted

quoted

+static void feature_amu_trap_activate(struct kvm_vcpu *vcpu)
+{
+       feature_trap_activate(vcpu, VCPU_CPTR_EL2, CPTR_EL2_TAM, 0);

Covers the CPTR flags for AMU, but as you mentioned, does not
explicitly clear HCR_AMVOFFEN.

In my understanding, clearing HCR_EL2.AMVOFFEN is not necessary as
CPTR_EL2.TAM == 1 traps the guest's accessing AMEVCNTR0<n>_EL0 and
AMEVCNTR1<n>_EL0 anyway (HCR_EL2.AMVOFFEN doesn't matter).
(Or is my understanding wrong ??)

You're right. However, I think they should be cleared first for
completeness. Also, if I understand correctly, AMVOFFEN is about
enabling and disabling virtualization of the registers, making
indirect reads of the virtual offset registers as zero, so it's not
just about trapping.

I understand that AMVOFFEN is making indirect reads of the
virtual offset registers as zero.  But, in my understanding,
enabling and disabling virtualization of the registers doesn't
matter as long as CPTR_EL2.TAM == 1 (a value of HCR_EL2.AMVOFFEN
doesn't change any behavior because the virtual offset registers
won't be used). So, I'm not too keen on adding that so far.

Thanks,
Reiji

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