Re: [PATCH 4/4] KVM: nVMX: Map enlightened VMCS upon restore when possible

[Vitaly Kuznetsov]

Paolo Bonzini <pbonzini@xxxxxxxxxx> writes:

> On 03/05/21 17:08, Vitaly Kuznetsov wrote:
>> It now looks like a bad idea to not restore eVMCS mapping directly from
>> vmx_set_nested_state(). The restoration path now depends on whether KVM
>> will continue executing L2 (vmx_get_nested_state_pages()) or will have to
>> exit to L1 (nested_vmx_vmexit()), this complicates error propagation and
>> diverges too much from the 'native' path when 'nested.current_vmptr' is
>> set directly from vmx_get_nested_state_pages().
>> 
>> The existing solution postponing eVMCS mapping also seems to be fragile.
>> In multiple places the code checks whether 'vmx->nested.hv_evmcs' is not
>> NULL to distinguish between eVMCS and non-eVMCS cases. All these checks
>> are 'incomplete' as we have a weird 'eVMCS is in use but not yet mapped'
>> state.
>> 
>> Also, in case vmx_get_nested_state() is called right after
>> vmx_set_nested_state() without executing the guest first, the resulting
>> state is going to be incorrect as 'KVM_STATE_NESTED_EVMCS' flag will be
>> missing.
>> 
>> Fix all these issues by making eVMCS restoration path closer to its
>> 'native' sibling by putting eVMCS GPA to 'struct kvm_vmx_nested_state_hdr'.
>> To avoid ABI incompatibility, do not introduce a new flag and keep the
>
> I'm not sure what is the disadvantage of not having a new flag.
>

Adding a new flag would make us backwards-incompatible both ways:

1) Migrating 'new' state to an older KVM will fail the

	if (kvm_state->hdr.vmx.flags & ~KVM_STATE_VMX_PREEMPTION_TIMER_DEADLINE)
	        return -EINVAL;

check.

2) Migrating 'old' state to 'new' KVM would make us support the old path
('KVM_REQ_GET_NESTED_STATE_PAGES') so the flag will still be 'optional'.

> Having two different paths with subtly different side effects however 
> seems really worse for maintenance.  We are already discussing in 
> another thread how to get rid of the check_nested_events side effects; 
> that might possibly even remove the need for patch 1, so it's at least 
> worth pursuing more than adding this second path.

I have to admit I don't fully like this solution either :-( In case we
make sure KVM_REQ_GET_NESTED_STATE_PAGES always gets handled the fix can
be omitted indeed, however, I still dislike the divergence and the fact
that 'if (vmx->nested.hv_evmcs)' checks scattered across the code are
not fully valid. E.g. how do we fix immediate KVM_GET_NESTED_STATE after
KVM_SET_NESTED_STATE without executing the vCPU problem?

>
> I have queued patch 1, but I'd rather have a kvm selftest for it.  It 
> doesn't seem impossible to have one...

Thank you, the band-aid solves a real problem. Let me try to come up
with a selftest for it.

>
> Paolo
>
>> original eVMCS mapping path through KVM_REQ_GET_NESTED_STATE_PAGES in
>> place. To distinguish between 'new' and 'old' formats consider eVMCS
>> GPA == 0 as an unset GPA (thus forcing KVM_REQ_GET_NESTED_STATE_PAGES
>> path). While technically possible, it seems to be an extremely unlikely
>> case.
>
>
>> Signed-off-by: Vitaly Kuznetsov<vkuznets@xxxxxxxxxx>
>

-- 
Vitaly