[PATCH] powerpc/64s: support nospectre_v2 cmdline option

[Christopher M Riedl]

> On May 5, 2019 at 9:32 PM Andrew Donnellan <ajd at linux.ibm.com> wrote:
> 
> 
> On 6/5/19 8:10 am, Christopher M. Riedl wrote:
> > Add support for disabling the kernel implemented spectre v2 mitigation
> > (count cache flush on context switch) via the nospectre_v2 cmdline
> > option.
> > 
> > Suggested-by: Michael Ellerman <mpe at ellerman.id.au>
> > Signed-off-by: Christopher M. Riedl <cmr at informatik.wtf>
> > ---
> > 
> >   arch/powerpc/kernel/security.c | 14 ++++++++++++--
> >   1 file changed, 12 insertions(+), 2 deletions(-)
> > 
> > diff --git a/arch/powerpc/kernel/security.c b/arch/powerpc/kernel/security.c
> > index b33bafb8fcea..f7c34745cd0f 100644
> > --- a/arch/powerpc/kernel/security.c
> > +++ b/arch/powerpc/kernel/security.c
> > @@ -28,7 +28,7 @@ static enum count_cache_flush_type count_cache_flush_type = COUNT_CACHE_FLUSH_NO
> >   bool barrier_nospec_enabled;
> >   static bool no_nospec;
> >   static bool btb_flush_enabled;
> > -#ifdef CONFIG_PPC_FSL_BOOK3E
> > +#if defined(CONFIG_PPC_FSL_BOOK3E) || defined(CONFIG_PPC_BOOK3S_64)
> >   static bool no_spectrev2;
> >   #endif
> >   
> > @@ -106,7 +106,7 @@ static __init int barrier_nospec_debugfs_init(void)
> >   device_initcall(barrier_nospec_debugfs_init);
> >   #endif /* CONFIG_DEBUG_FS */
> >   
> > -#ifdef CONFIG_PPC_FSL_BOOK3E
> > +#if defined(CONFIG_PPC_FSL_BOOK3E) || defined(CONFIG_PPC_BOOK3S_64)
> >   static int __init handle_nospectre_v2(char *p)
> >   {
> >   	no_spectrev2 = true;
> > @@ -114,6 +114,9 @@ static int __init handle_nospectre_v2(char *p)
> >   	return 0;
> >   }
> >   early_param("nospectre_v2", handle_nospectre_v2);
> > +#endif /* CONFIG_PPC_FSL_BOOK3E || CONFIG_PPC_BOOK3S_64 */
> > +
> > +#ifdef CONFIG_PPC_FSL_BOOK3E
> >   void setup_spectre_v2(void)
> >   {
> >   	if (no_spectrev2)
> > @@ -391,6 +394,13 @@ static void toggle_count_cache_flush(bool enable)
> >   
> >   void setup_count_cache_flush(void)
> >   {
> > +	if (no_spectrev2) {
> > +		if (security_ftr_enabled(SEC_FTR_BCCTRL_SERIALISED)
> > +		    || security_ftr_enabled(SEC_FTR_COUNT_CACHE_DISABLED))
> > +			pr_warn("Spectre v2 mitigations not under software control, can't disable\n");
> 
> If one of those ftrs is set, what's the impact of not calling 
> toggle_count_cache_flush()?
> 

The patchsite/callsite (kernel/entry_64.S:597) for flush_count_cache
inside _switch remains a nop.

Disassembly of vmlinux after build:
c00000000000e260:       00 00 23 f8     std     r1,0(r3)
c00000000000e264:       00 00 00 60     nop
c00000000000e268:       00 60 c0 3c     lis     r6,24576

Disassembly (xmon) after boot/during runtime in qemu:
c00000000000e260  f8230000	std     r1,0(r3)
c00000000000e264  4bffdb7d	bl      c00000000000bde0	# flush_count_cache+0x0/0x2420
c00000000000e268  3cc06000	lis     r6,24576

Disassembly (xmon) after boot/during runtime in qemu w/ nospectre_v2:
c00000000000e260  f8230000	std     r1,0(r3)
c00000000000e264  60000000	nop
c00000000000e268  3cc06000	lis     r6,24576

toggle_count_cache_flush() well uhh "toggles" the patchsite to either
contain a branch to the flush procedure or a nop.

> > +		return;
> > +	}
> > +
> >   	toggle_count_cache_flush(true);
> >   }
> >   
> > 
> 
> -- 
> Andrew Donnellan              OzLabs, ADL Canberra
> ajd at linux.ibm.com             IBM Australia Limited
>