Re: [PATCH v4 2/3] xen/oprofile: use NMI continuation for sending virq to guest

[Jan Beulich <jbeulich@xxxxxxxx>]

On 12.11.2020 11:48, Jürgen Groß wrote:
> On 12.11.20 11:23, Jan Beulich wrote:
>> On 11.11.2020 16:48, Jürgen Groß wrote:
>>> On 11.11.20 16:45, Jan Beulich wrote:
>>>> On 09.11.2020 10:50, Juergen Gross wrote:
>>>>>    static int nmi_callback(const struct cpu_user_regs *regs, int cpu)
>>>>>    {
>>>>>           int xen_mode, ovf;
>>>>>    
>>>>>           ovf = model->check_ctrs(cpu, &cpu_msrs[cpu], regs);
>>>>>           xen_mode = ring_0(regs);
>>
>> Unrelated to the patch here (i.e. just as an observation), this
>> use of ring_0() looks bogus when the NMI occurred in HVM guest
>> mode.
> 
> An NMI in an HVM guest due to oprofile would be a VMEXIT with NMI
> reason, or just be handled completely inside the guest, right?

Yes, and in the former case for VMX it would be handed on to do_nmi(),
with the guest register state. For SVM it would get handled on the
next STGI, i.e. would indeed never surface from HVM guest mode.

> I don't see how this test should ever result in xen_mode being
> false for an HVM guest.

I think, because of hvm_invalidate_regs_fields(), on VMX it would be
consistently true in release builds and consistently false in debug
ones.

Jan