Re: [PATCH] x86/Xen: make use of IBPB controlling VM assist

[Boris Ostrovsky <boris.ostrovsky@xxxxxxxxxx>]

On 2/15/23 3:31 AM, Jan Beulich wrote:
> On 15.02.2023 01:07, Boris Ostrovsky wrote:
> > On 2/14/23 6:53 PM, Boris Ostrovsky wrote:
> > > On 2/14/23 11:13 AM, Jan Beulich wrote:
> > >
> > > > --- a/arch/x86/kernel/cpu/bugs.c
> > > > +++ b/arch/x86/kernel/cpu/bugs.c
> > > > @@ -18,6 +18,8 @@
> > > >    #include <linux/pgtable.h>
> > > >    #include <linux/bpf.h>
> > > >    +#include <xen/xen.h>
> > > > +
> > > >    #include <asm/spec-ctrl.h>
> > > >    #include <asm/cmdline.h>
> > > >    #include <asm/bugs.h>
> > > > @@ -32,6 +34,7 @@
> > > >    #include <asm/intel-family.h>
> > > >    #include <asm/e820/api.h>
> > > >    #include <asm/hypervisor.h>
> > > > +#include <asm/xen/hypervisor.h>
> > > >    #include <asm/tlbflush.h>
> > > >      #include "cpu.h"
> > > > @@ -934,7 +937,8 @@ do_cmd_auto:
> > > >            break;
> > > >          case RETBLEED_MITIGATION_IBPB:
> > > > -        setup_force_cpu_cap(X86_FEATURE_ENTRY_IBPB);
> > > > +        if (!xen_pv_domain() || xen_vm_assist_ibpb(true))
> > > Is this going to compile without CONFIG_XEN?
> Yes. The declaration of xen_vm_assist_ibpb() is visible (satisfying
> the compiler) and DCE will eliminate the call to the function due to
> xen_pv_domain() being constant "false" in that case, avoiding any
> linking issues. The interesting case here really is building with
> XEN but without XEN_PV: That's why I needed to put the function in
> enlighten.c. This wouldn't be needed if xen_pv_domain() was also
> constant "false" in that case (just like xen_pvh_domain() is when
> !XEN_PVH).
>
> > > I also think these two conditions should be wrapped into something to limit 
> > > exposure of non-Xen code to Xen-specific primitives.
> I would have done so, if I had any halfway sensible idea on how to
> go about doing so in this particular case. In the absence of that it
> looked okay-ish to me to reference Xen functions directly here.
>
> > Oh, and this needs x86 maintainers.
> Eventually yes. But I would prefer to sort the above question first
> (which I'm sure would have been raised by them, in perhaps more
> harsh a way), hence the initially limited exposure.
I also think there is a bit of a disconnect between how the mitigation is reported in the 
log/sysfs (retbleed_mitigation is RETBLEED_MITIGATION_IBPB, so "Mitigation: 
IBPB") and, for example, lscpu (since X86_FEATURE_ENTRY_IBPB is not set anymore).


-boris