Re: [PATCH v5 4/5] KVM: VMX: Allow direct access to MSR_IA32_SPEC_CTRL

[KarimAllah Ahmed <karahmed@amazon.com>]

On 01/31/2018 09:18 PM, Jim Mattson wrote:
> On Wed, Jan 31, 2018 at 12:01 PM, KarimAllah Ahmed <karahmed@amazon.com> wrote:
> 
>> but save_spec_ctrl_on_exit is also set for L2 write. So once L2 writes
>> to it, this condition will be true and then the bitmap will be updated.
> 
> So if L1 or any L2 writes to the MSR, then save_spec_ctrl_on_exit is
> set to true, even if the MSR permission bitmap for a particular VMCS
> *doesn't* allow the MSR to be written without an intercept. That's
> functionally correct, but inefficient. It seems to me that
> save_spec_ctrl_on_exit should indicate whether or not the *current*
> MSR permission bitmap allows unintercepted writes to IA32_SPEC_CTRL.
> To that end, perhaps save_spec_ctrl_on_exit rightfully belongs in the
> loaded_vmcs structure, alongside the msr_bitmap pointer that it is
> associated with. For vmcs02, nested_vmx_merge_msr_bitmap() should set
> the vmcs02 save_spec_ctrl_on_exit based on (a) whether L0 is willing
> to yield the MSR to L1, and (b) whether L1 is willing to yield the MSR
> to L2.

I actually got rid of this save_spec_ctrl_on_exit variable and replaced
it with another variable like the one suggested for IBPB. Just to avoid
doing an expensive guest_cpuid_has. Now I peak instead in the MSR bitmap
to figure out if this MSR was supposed to be intercepted or not. This
test should provide a similar semantics to save_spec_ctrl_on_exit.

Anyway, cleaning up/testing now and will post a new version.
Amazon Development Center Germany GmbH
Berlin - Dresden - Aachen
main office: Krausenstr. 38, 10117 Berlin
Geschaeftsfuehrer: Dr. Ralf Herbrich, Christian Schlaeger
Ust-ID: DE289237879
Eingetragen am Amtsgericht Charlottenburg HRB 149173 B