On Thu, Jul 14, 2022 at 01:42:11PM +0200, Peter Zijlstra wrote: > On Thu, Jul 14, 2022 at 08:17:26AM -0300, Thadeu Lima de Souza Cascardo wrote: > > On Wed, Jul 13, 2022 at 10:32:37PM -0700, Pawan Gupta wrote: > > > Currently spectre_v2=ibrs forces write to MSR_IA32_SPEC_CTRL at every > > > entry and exit. On Enhanced IBRS parts setting MSR_IA32_SPEC_CTRL[IBRS] > > > only once at bootup is sufficient. MSR write at every kernel entry/exit > > > incur unnecessary penalty that can be avoided. > > > > > > When Enhanced IBRS feature is present, switch from "ibrs" to "auto" mode > > > so that appropriate mitigation is selected. > > > > > > Fixes: 7c693f54c873 ("x86/speculation: Add spectre_v2=ibrs option to support Kernel IBRS") > > > Cc: stable@xxxxxxxxxxxxxxx # 5.10+ > > > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx> > > > --- > > > arch/x86/kernel/cpu/bugs.c | 6 ++++++ > > > 1 file changed, 6 insertions(+) > > > > > > diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c > > > index 0dd04713434b..7d7ebfdfbeda 100644 > > > --- a/arch/x86/kernel/cpu/bugs.c > > > +++ b/arch/x86/kernel/cpu/bugs.c > > > @@ -1303,6 +1303,12 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void) > > > return SPECTRE_V2_CMD_AUTO; > > > } > > > > > > + if (cmd == SPECTRE_V2_CMD_IBRS && boot_cpu_has(X86_FEATURE_IBRS_ENHANCED)) { > > > + pr_err("%s selected but CPU supports Enhanced IBRS. Switching to AUTO select\n", > > > + mitigation_options[i].option); > > > + return SPECTRE_V2_CMD_AUTO; > > > + } > > > + > > > spec_v2_print_cond(mitigation_options[i].option, > > > mitigation_options[i].secure); > > > return cmd; > > > > > > base-commit: 72a8e05d4f66b5af7854df4490e3135168694b6b > > > -- > > > 2.35.3 > > > > > > > > > > Shouldn't we just use the mitigation the user asked for if it is still > > possible? We could add the warning advising the user that a different > > mitigation could be used instead with less penalty, but if the user asked for > > IBRS and that is available, it should be used. > > > > One of the reasons for that is testing. I know it was useful enough for me and > > it helped me find some bugs. > > Yeah this; if the user asks for IBRS, we should give him IBRS. I hate > the 'I know better, let me change that for you' mentality. eIBRS CPUs don't even have legacy IBRS so I don't see how this is even possible. -- Josh