Re: [PATCH] x86/perf: Fix guest_get_msrs static call if there is no PMU

[Like Xu <like.xu@linux.intel.com>]

On 2021/3/8 15:12, Dmitry Vyukov wrote:
> On Mon, Mar 8, 2021 at 3:26 AM Xu, Like <like.xu@intel.com> wrote:
>>
>> On 2021/3/6 6:33, Sean Christopherson wrote:
>>> Handle a NULL x86_pmu.guest_get_msrs at invocation instead of patching
>>> in perf_guest_get_msrs_nop() during setup.  If there is no PMU, setup
>>
>> "If there is no PMU" ...
>>
>> How to set up this kind of environment,
>> and what changes are needed in .config or boot parameters ?
> 
> Hi Xu,
> 
> This can be reproduced in qemu with "-cpu max,-pmu" flag using this reproducer:
> https://groups.google.com/g/syzkaller-bugs/c/D8eHw3LIOd0/m/L2G0lVkVBAAJ

Sorry, I couldn't reproduce any VMX abort with "-cpu max,-pmu".
Doe this patch fix this "unexpected kernel reboot" issue ?

If so, you may add "Tested-by" for more attention.

> 
>>> bails before updating the static calls, leaving x86_pmu.guest_get_msrs
>>> NULL and thus a complete nop.
>>
>>> Ultimately, this causes VMX abort on
>>> VM-Exit due to KVM putting random garbage from the stack into the MSR
>>> load list.
>>>
>>> Fixes: abd562df94d1 ("x86/perf: Use static_call for x86_pmu.guest_get_msrs")
>>> Cc: Like Xu <like.xu@linux.intel.com>
>>> Cc: Paolo Bonzini <pbonzini@redhat.com>
>>> Cc: Jim Mattson <jmattson@google.com>
>>> Cc: kvm@vger.kernel.org
>>> Reported-by: Dmitry Vyukov <dvyukov@google.com>
>>> Signed-off-by: Sean Christopherson <seanjc@google.com>
>>> ---
>>>    arch/x86/events/core.c | 16 +++++-----------
>>>    1 file changed, 5 insertions(+), 11 deletions(-)
>>>
>>> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
>>> index 6ddeed3cd2ac..ff874461f14c 100644
>>> --- a/arch/x86/events/core.c
>>> +++ b/arch/x86/events/core.c
>>> @@ -671,7 +671,11 @@ void x86_pmu_disable_all(void)
>>>
>>>    struct perf_guest_switch_msr *perf_guest_get_msrs(int *nr)
>>>    {
>>> -     return static_call(x86_pmu_guest_get_msrs)(nr);
>>> +     if (x86_pmu.guest_get_msrs)
>>> +             return static_call(x86_pmu_guest_get_msrs)(nr);
>>
>> How about using "static_call_cond" per commit "452cddbff7" ?
>>
>>> +
>>> +     *nr = 0;
>>> +     return NULL;
>>>    }
>>>    EXPORT_SYMBOL_GPL(perf_guest_get_msrs);
>>>
>>> @@ -1944,13 +1948,6 @@ static void _x86_pmu_read(struct perf_event *event)
>>>        x86_perf_event_update(event);
>>>    }
>>>
>>> -static inline struct perf_guest_switch_msr *
>>> -perf_guest_get_msrs_nop(int *nr)
>>> -{
>>> -     *nr = 0;
>>> -     return NULL;
>>> -}
>>> -
>>>    static int __init init_hw_perf_events(void)
>>>    {
>>>        struct x86_pmu_quirk *quirk;
>>> @@ -2024,9 +2021,6 @@ static int __init init_hw_perf_events(void)
>>>        if (!x86_pmu.read)
>>>                x86_pmu.read = _x86_pmu_read;
>>>
>>> -     if (!x86_pmu.guest_get_msrs)
>>> -             x86_pmu.guest_get_msrs = perf_guest_get_msrs_nop;
>>> -
>>>        x86_pmu_static_call_update();
>>>
>>>        /*
>>