Re: [PATCH v4 2/6] KVM: x86: nSVM: implement nested LBR virtualization

[Maxim Levitsky]

On Mon, 2022-03-28 at 19:12 +0200, Paolo Bonzini wrote:
> On 3/27/22 17:12, Maxim Levitsky wrote:
> > - with LBR virtualization supported, the guest can set this msr to any value
> > as long as it doesn't set reserved bits and then read back the written value,
> > but it is not used by the CPU, unless LBR bit is set in MSR_IA32_DEBUGCTLMSR,
> > because only then LBR virtualization is enabled, which makes the CPU
> > load the guest value on VM entry.
> >   
> > This means that MSR_IA32_DEBUGCTLMSR.BTF will magically start working when
> > MSR_IA32_DEBUGCTLMSR.LBR is set as well, and will not work otherwise.
> 
> That can be fixed by context-switching DEBUGCTLMSR by hand when LBR=0 && 
> BTF=1.  Would you like to give it a shot?
> 
> Paolo
> 
Yep exactly, I didn't do that yet only because mypatches didn't made it worse,
so I wanted to do this in a separate patch (+unit test), and it kind of
went to my backlog. I'll do that soon.

Best regards,
	Maxim Levitsky