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

[Eduardo Habkost]

On Thu, Aug 30, 2018 at 12:22:10PM +0800, Robert Hoo wrote:
> 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)

Making kvm_arch_get_supported_msr_feature() return 0 if the
capability isn't available (except for RSBA) seems to be enough
to ensure we do the right thing on both cases (1 and 2 above).

Now, if KVM_GET_MSR_FEATURE_INDEX_LIST returns an unexpected
error, it sounds like an exceptional situation that can justify a
fatal error (or at least a warning).

-- 
Eduardo