Re: [Qemu-devel] [PATCH v3 2/3] kvm: Add support to KVM_GET_MSR_FEATURE_INDEX_LIST and KVM_GET_MSRS system ioctl

[Robert Hoo]

On Thu, 2018-08-23 at 14:11 -0300, Eduardo Habkost wrote:
> On Thu, Aug 23, 2018 at 02:28:28PM +0800, Robert Hoo wrote:
> > On Sat, 2018-08-18 at 12:05 -0300, Eduardo Habkost wrote:
> [...]
> > > We don't want QEMU to refuse to run if the kernel doesn't have
> > > KVM_CAP_GET_MSR_FEATURES.  We can treat missing capability as
> > > equivalent to returning an empty list of MSRs.
> > Yes. I'll let caller (kvm_arch_init) ignore the return value but a
> > simple warning.
> 
> Warnings tend to be ignored and are generally a sign that QEMU
> isn't doing the right thing.  Sometimes we have no choice, but I
> don't think that's the case here.
> 
> As far as I can see, we have only two possibilities here:
> 1) The host can run a VM that behaves exactly as requested on the
>    command-line (no warning required).
> 2) The host can't run the requested configuration (fatal error,
>    not a warning).
> 
I mean the kvm_arch_init() --> kvm_get_supported_feature_msrs(), when
kvm_get_supported_feature_msrs() returns error, can we ignore it?
The cases of kvm_get_supported_feature_msrs() returning error:
1) underlying KVM doesn't support GET_MSR_FEATURES capability.
2) error in KVM_GET_MSR_FEATURE_INDEX_LIST.

given the principle you guided before, I think we can ignore above error
cases, and later kvm_arch_get_supported_msr_feature() would always
return 0 (of course, except those special cases like
IA32_ARCH_CAPABILITIES.RSBA)