Re: [RFC PATCH 1/2] x86, lib, xenpv: Add WBNOINVD helper functions

[Kevin Loughlin <kevinloughlin@xxxxxxxxxx>]

On Mon, Dec 2, 2024 at 5:28 PM Andrew Cooper <andrew.cooper3@xxxxxxxxxx> wrote:
>
> On 03/12/2024 12:59 am, Kevin Loughlin wrote:
> > diff --git a/arch/x86/include/asm/paravirt.h 
> > b/arch/x86/include/asm/paravirt.h
> > index d4eb9e1d61b8..c040af2d8eff 100644
> > --- a/arch/x86/include/asm/paravirt.h
> > +++ b/arch/x86/include/asm/paravirt.h
> > @@ -187,6 +187,13 @@ static __always_inline void wbinvd(void)
> >       PVOP_ALT_VCALL0(cpu.wbinvd, "wbinvd", ALT_NOT_XEN);
> >  }
> >
> > +extern noinstr void pv_native_wbnoinvd(void);
> > +
> > +static __always_inline void wbnoinvd(void)
> > +{
> > +     PVOP_ALT_VCALL0(cpu.wbnoinvd, "wbnoinvd", ALT_NOT_XEN);
> > +}
>
> Given this, ...
>
> > diff --git a/arch/x86/kernel/paravirt.c b/arch/x86/kernel/paravirt.c
> > index fec381533555..a66b708d8a1e 100644
> > --- a/arch/x86/kernel/paravirt.c
> > +++ b/arch/x86/kernel/paravirt.c
> > @@ -149,6 +154,7 @@ struct paravirt_patch_template pv_ops = {
> >       .cpu.write_cr0          = native_write_cr0,
> >       .cpu.write_cr4          = native_write_cr4,
> >       .cpu.wbinvd             = pv_native_wbinvd,
> > +     .cpu.wbnoinvd           = pv_native_wbnoinvd,
> >       .cpu.read_msr           = native_read_msr,
> >       .cpu.write_msr          = native_write_msr,
> >       .cpu.read_msr_safe      = native_read_msr_safe,
>
> this, and ...
>
> > diff --git a/arch/x86/xen/enlighten_pv.c b/arch/x86/xen/enlighten_pv.c
> > index d6818c6cafda..a5c76a6f8976 100644
> > --- a/arch/x86/xen/enlighten_pv.c
> > +++ b/arch/x86/xen/enlighten_pv.c
> > @@ -1162,6 +1162,7 @@ static const typeof(pv_ops) xen_cpu_ops __initconst = 
> > {
> >               .write_cr4 = xen_write_cr4,
> >
> >               .wbinvd = pv_native_wbinvd,
> > +             .wbnoinvd = pv_native_wbnoinvd,
> >
> >               .read_msr = xen_read_msr,
> >               .write_msr = xen_write_msr,
>
> this, what is the point having a paravirt hook which is wired to
> native_wbnoinvd() in all cases?
>
> That just seems like overhead for overhead sake.

I'm mirroring what's done for WBINVD here, which was changed to a
paravirt hook in 10a099405fdf ("cpuidle, xenpv: Make more PARAVIRT_XXL
noinstr clean") in order to avoid calls out to instrumented code as
described in the commit message in more detail. I believe a hook is
similarly required for WBNOINVD, but please let me know if you
disagree. Thanks!