Re: [RFC PATCH 2/2] KVM: VMX: Stop/resume host PT before/after VM entry when PT_MODE_HOST_GUEST

[Xiaoyao Li]

On 8/25/2022 11:34 PM, Sean Christopherson wrote:
> On Thu, Aug 25, 2022, Xiaoyao Li wrote:
> > diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> > index d7f8331d6f7e..3e9ce8f600d2 100644
> > --- a/arch/x86/kvm/vmx/vmx.c
> > +++ b/arch/x86/kvm/vmx/vmx.c
> > @@ -38,6 +38,7 @@
> >   #include <asm/fpu/api.h>
> >   #include <asm/fpu/xstate.h>
> >   #include <asm/idtentry.h>
> > +#include <asm/intel_pt.h>
> >   #include <asm/io.h>
> >   #include <asm/irq_remapping.h>
> >   #include <asm/kexec.h>
> > @@ -1128,13 +1129,19 @@ static void pt_guest_enter(struct vcpu_vmx *vmx)
> >   	if (vmx_pt_mode_is_system())
> >   		return;
> >   
> > +	/*
> > +	 * Stop Intel PT on host to avoid vm-entry failure since
> > +	 * VM_ENTRY_LOAD_IA32_RTIT_CTL is set
> > +	 */
> > +	intel_pt_stop();
> > +
> >   	/*
> >   	 * GUEST_IA32_RTIT_CTL is already set in the VMCS.
> >   	 * Save host state before VM entry.
> >   	 */
> >   	rdmsrl(MSR_IA32_RTIT_CTL, vmx->pt_desc.host.ctl);
>
> KVM's manual save/restore of MSR_IA32_RTIT_CTL should be dropped.  

No. It cannot. Please see below.

> If PT/RTIT can
> trace post-VMXON, then intel_pt_stop() will disable tracing and intel_pt_resume()
> will restore the host's desired value.

intel_pt_stop() and intel_pt_resume() touches host's RTIT_CTL only when 
host enables/uses Intel PT. Otherwise, they're just noop. In this case, 
we cannot assume host's RTIT_CTL is zero (only the RTIT_CTL.TraceEn is 
0). After VM-exit, RTIT_CTL is cleared, we need to restore it.

> >   	if (vmx->pt_desc.guest.ctl & RTIT_CTL_TRACEEN) {
> > -		wrmsrl(MSR_IA32_RTIT_CTL, 0);
> > +		/* intel_pt_stop() ensures RTIT_CTL.TraceEn is zero */
> >   		pt_save_msr(&vmx->pt_desc.host, vmx->pt_desc.num_address_ranges);
>
> Isn't this at risk of the same corruption?  What prevents a PT NMI that arrives
> after this point from changing other RTIT MSRs, thus causing KVM to restore the
> wrong values?

intel_pt_stop() -> pt_event_stop() will do

	WRITE_ONCE(pt->handle_nmi, 0);

which ensure PT NMI handler as noop that at the beginning of 
intel_pt_interrupt():

	if (!READ_ONCE(pt->handle_nmi))
		return;

> >   		pt_load_msr(&vmx->pt_desc.guest, vmx->pt_desc.num_address_ranges);
> >   	}
> > @@ -1156,6 +1163,8 @@ static void pt_guest_exit(struct vcpu_vmx *vmx)
> >   	 */
> >   	if (vmx->pt_desc.host.ctl)
> >   		wrmsrl(MSR_IA32_RTIT_CTL, vmx->pt_desc.host.ctl);
> > +
> > +	intel_pt_resume();
> >   }
> >   
> >   void vmx_set_host_fs_gs(struct vmcs_host_state *host, u16 fs_sel, u16 gs_sel,
> > --
> > 2.27.0