> On May 5, 2019 at 9:32 PM Andrew Donnellan <a...@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 <m...@ellerman.id.au>
> > Signed-off-by: Christopher M. Riedl <c...@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
> a...@linux.ibm.com IBM Australia Limited
>
