Re: [PATCH v3 2/8] x86/split_lock: Ensure X86_FEATURE_SPLIT_LOCK_DETECT means the existence of feature

Xiaoyao Li
2020-03-04T01:49:23Z

On 3/4/2020 3:41 AM, Sean Christopherson wrote:
> On Tue, Mar 03, 2020 at 10:55:24AM -0800, Sean Christopherson wrote:
>> On Thu, Feb 06, 2020 at 03:04:06PM +0800, Xiaoyao Li wrote:
>>> When flag X86_FEATURE_SPLIT_LOCK_DETECT is set, it should ensure the
>>> existence of MSR_TEST_CTRL and MSR_TEST_CTRL.SPLIT_LOCK_DETECT bit.
>>
>> The changelog confused me a bit. "When flag X86_FEATURE_SPLIT_LOCK_DETECT
>> is set" makes it sound like the logic is being applied after the feature
>> bit is set. Maybe something like:
>>
>> ```
>> Verify MSR_TEST_CTRL.SPLIT_LOCK_DETECT can be toggled via WRMSR prior to
>> setting the SPLIT_LOCK_DETECT feature bit so that runtime consumers,
>> e.g. KVM, don't need to worry about WRMSR failure.
>> ```
>>
>>> Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
>>> ---
>>> arch/x86/kernel/cpu/intel.c | 41 +++++++++++++++++++++----------------
>>> 1 file changed, 23 insertions(+), 18 deletions(-)
>>>
>>> diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
>>> index 2b3874a96bd4..49535ed81c22 100644
>>> --- a/arch/x86/kernel/cpu/intel.c
>>> +++ b/arch/x86/kernel/cpu/intel.c
>>> @@ -702,7 +702,8 @@ static void init_intel(struct cpuinfo_x86 *c)
>>> if (tsx_ctrl_state == TSX_CTRL_DISABLE)
>>> tsx_disable();
>>>
>>> - split_lock_init();
>>> + if (boot_cpu_has(X86_FEATURE_SPLIT_LOCK_DETECT))
>>> + split_lock_init();
>>> }
>>>
>>> #ifdef CONFIG_X86_32
>>> @@ -986,9 +987,26 @@ static inline bool match_option(const char *arg, int arglen, const char *opt)
>>>
>>> static void __init split_lock_setup(void)
>>> {
>>> + u64 test_ctrl_val;
>>> char arg[20];
>>> int i, ret;
>>> + /*
>>> + * Use the "safe" versions of rdmsr/wrmsr here to ensure MSR_TEST_CTRL
>>> + * and MSR_TEST_CTRL.SPLIT_LOCK_DETECT bit do exist. Because there may
>>> + * be glitches in virtualization that leave a guest with an incorrect
>>> + * view of real h/w capabilities.
>>> + */
>>> + if (rdmsrl_safe(MSR_TEST_CTRL, &test_ctrl_val))
>>> + return;
>>> +
>>> + if (wrmsrl_safe(MSR_TEST_CTRL,
>>> + test_ctrl_val | MSR_TEST_CTRL_SPLIT_LOCK_DETECT))
>>> + return;
>>> +
>>> + if (wrmsrl_safe(MSR_TEST_CTRL, test_ctrl_val))
>>> + return;a
>>
>> Probing the MSR should be skipped if SLD is disabled in sld_options, i.e.
>> move this code (and setup_force_cpu_cap() etc...) down below the
>> match_option() logic. The above would temporarily enable SLD even if the
>> admin has explicitly disabled it, e.g. makes the kernel param useless for
>> turning off the feature due to bugs.
>
> Hmm, but this prevents KVM from exposing SLD to a guest when it's off in
> the kernel, which would be a useful debug/testing scenario.
>
> Maybe add another SLD state to forcefully disable SLD? That way the admin
> can turn of SLD in the host kernel but still allow KVM to expose it to its
> guests. E.g.

I don't think we need do this.

IMO, this a the bug of split_lock_init(), which assume the initial value
of MSR_TEST_CTRL is zero, at least bit SPLIT_LOCK of which is zero.
This is problem, it's possible that BIOS has set this bit.

split_lock_setup() here, is to check if the feature really exists. So
probing MSR_TEST_CTRL and bit MSR_TEST_CTRL_SPLIT_LOCK_DETECT here. If
there all exist, setup_force_cpu_cap(X86_FEATURE_SPLIT_LOCK_DETECT) to
indicate feature does exist.
Only when feature exists, there is a need to parse the command line
config of split_lock_detect.

In split_lock_init(), we should explicitly clear
MSR_TEST_CTRL_SPLIT_LOCK_DETECT bit if sld_off.

> static const struct {
> const char *option;
> enum split_lock_detect_state state;
> } sld_options[] __initconst = {
> { "disable", sld_disable },
> { "off", sld_off },
> { "warn", sld_warn },
> { "fatal", sld_fatal },
> };
>
>
> Then the new setup() becomes:
>
> static void __init split_lock_setup(void)
> {
> u64 test_ctrl_val;
> char arg[20];
> int i, ret;
>
> sld_state = sld_warn;
>
> ret = cmdline_find_option(boot_command_line, "split_lock_detect",
> arg, sizeof(arg));
> if (ret >= 0) {
> for (i = 0; i < ARRAY_SIZE(sld_options); i++) {
> if (match_option(arg, ret, sld_options[i].option)) {
> sld_state = sld_options[i].state;
> break;
> }
> }
> }
>
> if (sld_state == sld_disable)
> goto log_sld;
>
> /*
> * Use the "safe" versions of rdmsr/wrmsr here to ensure MSR_TEST_CTRL
> * and MSR_TEST_CTRL.SPLIT_LOCK_DETECT bit do exist. Because there may
> * be glitches in virtualization that leave a guest with an incorrect
> * view of real h/w capabilities.
> */
> if (rdmsrl_safe(MSR_TEST_CTRL, &test_ctrl_val))
> goto sld_broken;
>
> if (wrmsrl_safe(MSR_TEST_CTRL,
> test_ctrl_val | MSR_TEST_CTRL_SPLIT_LOCK_DETECT))
> goto sld_broken;
>
> if (wrmsrl_safe(MSR_TEST_CTRL, test_ctrl_val))
> goto sld_broken;
>
> setup_force_cpu_cap(X86_FEATURE_SPLIT_LOCK_DETECT);
>
> log_sld:
> switch (sld_state) {
> case sld_disable:
> pr_info("split_lock detection disabled\n");
> break;
> case sld_off:
> pr_info("split_lock detection off in kernel\n");
> break;
> case sld_warn:
> pr_info("warning about user-space split_locks\n");
> break;
> case sld_fatal:
> pr_info("sending SIGBUS on user-space split_locks\n");
> break;
> }
>
> return;
>
> sld_broken:
> sld_state = sld_disable;
> pr_err("split_lock detection disabled, MSR access faulted\n");
> }
>
>> And with that, IMO failing any of RDMSR/WRSMR here warrants a pr_err().
>> The CPU says it supports split lock and the admin hasn't explicitly turned
>> it off, so failure to enable should be logged.
>>
>>> +
>>> setup_force_cpu_cap(X86_FEATURE_SPLIT_LOCK_DETECT);
>>> sld_state = sld_warn;
>>>
>>> @@ -1022,24 +1040,19 @@ static void __init split_lock_setup(void)
>>> * Locking is not required at the moment because only bit 29 of this
>>> * MSR is implemented and locking would not prevent that the operation
>>> * of one thread is immediately undone by the sibling thread.
>>> - * Use the "safe" versions of rdmsr/wrmsr here because although code
>>> - * checks CPUID and MSR bits to make sure the TEST_CTRL MSR should
>>> - * exist, there may be glitches in virtualization that leave a guest
>>> - * with an incorrect view of real h/w capabilities.
>>> */
>>> -static bool __sld_msr_set(bool on)
>>> +static void __sld_msr_set(bool on)
>>> {
>>> u64 test_ctrl_val;
>>>
>>> - if (rdmsrl_safe(MSR_TEST_CTRL, &test_ctrl_val))
>>> - return false;
>>> + rdmsrl(MSR_TEST_CTRL, test_ctrl_val);
>>>
>>> if (on)
>>> test_ctrl_val |= MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
>>> else
>>> test_ctrl_val &= ~MSR_TEST_CTRL_SPLIT_LOCK_DETECT;
>>>
>>> - return !wrmsrl_safe(MSR_TEST_CTRL, test_ctrl_val);
>>> + wrmsrl(MSR_TEST_CTRL, test_ctrl_val);
>>> }
>>>
>>> static void split_lock_init(void)
>>> @@ -1047,15 +1060,7 @@ static void split_lock_init(void)
>>> if (sld_state == sld_off)
>>> return;
>>>
>>> - if (__sld_msr_set(true))
>>> - return;
>>> -
>>> - /*
>>> - * If this is anything other than the boot-cpu, you've done
>>> - * funny things and you get to keep whatever pieces.
>>> - */
>>> - pr_warn("MSR fail -- disabled\n");
>>> - sld_state = sld_off;
>>> + __sld_msr_set(true);
>>> }
>>>
>>> bool handle_user_split_lock(unsigned long ip)
>>> --
>>> 2.23.0
>>>