Re: [PATCH] Use WARN_ON_ONCE for missing X86_FEATURE_NRIPS

[Bandan Das <bsd@redhat.com>]

Joerg Roedel <joro@8bytes.org> writes:

> On Tue, Oct 06, 2015 at 01:59:27PM -0400, Bandan Das wrote:
>> Joerg Roedel <joro@8bytes.org> writes:
>> >
>> > So svm->vmcb->control.next_rip is only written by hardware or in
>> > svm_check_intercept(). Both cases write only to this field, if the
>> > hardware supports X86_FEATURE_NRIPS. The write in nested_svm_vmexit only
>> 
>> Not until commit f104765b4f81fd74d69e0eb161e89096deade2db. So, an older L1
>> kernel will trigger it.
>
> But we don't care if L1 writes something into its own next_rip, as we
> never read this value from its VMCB. We only copy the next_rip value we
> get from our shadow-vmcb to it on an emulated vmexit. So I still don't
> understand what triggers the reported problem or why the WARN_ON is
> necessary.

Ok, looks like I am making some incorrect "vmx" assumptions here. What happens
when we exit from L2 to L0, arent' we looking at the VMCB L1 is using to run
L2 ? Wouldn't that trigger the warning if the host processor does not support
nrips and the field is set ?


>
> 	Joerg