On Fri, 27 Aug 2021 16:06:16 +0200
Claudio Imbrenda [off-list ref] wrote:

On Fri, 27 Aug 2021 14:54:29 +0200
Halil Pasic [off-list ref] wrote:

quoted

While in practice vcpu->vcpu_idx ==  vcpu->vcp_id is often true,

s/vcp_id/vcpu_id/

quoted

it may not always be, and we must not rely on this.  

why?

maybe add a simple explanation of why vcpu_idx and vcpu_id can be
different, namely:
KVM decides the vcpu_idx, userspace decides the vcpu_id, thus the two
might not match 

Not sure that is a good explanation. A quote from
Documentation/virt/kvm/api.rst:

"""
4.7 KVM_CREATE_VCPU
-------------------

:Capability: basic
:Architectures: all
:Type: vm ioctl
:Parameters: vcpu id (apic id on x86)
:Returns: vcpu fd on success, -1 on error
                
This API adds a vcpu to a virtual machine. No more than max_vcpus may be added.
The vcpu id is an integer in the range [0, max_vcpu_id).
        
The recommended max_vcpus value can be retrieved using the KVM_CAP_NR_VCPUS of
the KVM_CHECK_EXTENSION ioctl() at run-time.
The maximum possible value for max_vcpus can be retrieved using the
KVM_CAP_MAX_VCPUS of the KVM_CHECK_EXTENSION ioctl() at run-time.
"""

Based on that and a quick look at the code, it looks to me like the
set of valid vcpu_id values are a subset of the range of vcpu_idx-es,
i.e. that kvm could decide to choose vcpu_id for the value of vcpu_idx.
I don't think it should, but it could. Were the set of valid vcpu_id
values not a subset of the set of supported vcpu_idx values, then one
could argue that this is why.

I didn't want to get into explaining the why, I just wanted to state the
fact.

quoted

Currently kvm->arch.idle_mask is indexed by vcpu_id, which implies
that code like
for_each_set_bit(vcpu_id, kvm->arch.idle_mask, online_vcpus) {
                vcpu = kvm_get_vcpu(kvm, vcpu_id);  

you can also add a sentence to clarify that kvm_get_vcpu expects an
vcpu_idx, not an vcpu_id.

maybe ...

quoted

		do_stuff(vcpu);
}
is not legit. The trouble is, we do actually use kvm->arch.idle_mask

... s/legit\./legit, because kvm_get_vcpu() expects a vcpu_idx and not a
vcpu_id.

But I agree kvm_get_vcpu(kvm, vcpu_id); does not scream BUG at me while
kvm_get_vcpu_by_idx(kvm, vcpu_id) would look much more suspicious.

[..]

otherwise looks good to me.

Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>

Thanks for your reveiew!

Halil

[..]