Messages in this thread | | | Date | Thu, 14 Jul 2022 07:59:22 -0700 | From | Pawan Gupta <> | Subject | Re: [PATCH] x86/bugs: Switch to "auto" when "ibrs" selected on Enhanced IBRS parts |
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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@vger.kernel.org # 5.10+ >> > Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com> >> > --- >> > 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. > >If you want to do something, print a warning.
Fair enough, I will change that to a warning.
Thanks, Pawan
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