Re: [PATCH 1/4] x86/reboot: Unconditionally define cpu_emergency_virt_cb typedef

["Huang, Kai" <kai.huang@intel.com>]

On 14/05/2024 10:44 am, Huang, Kai wrote:
> 
> 
> On 14/05/2024 4:01 am, Sean Christopherson wrote:
>> On Mon, May 13, 2024, Kai Huang wrote:
>>> On Thu, 2024-04-25 at 16:39 -0700, Sean Christopherson wrote:
>>>> Define cpu_emergency_virt_cb even if the kernel is being built 
>>>> without KVM
>>>> support so that KVM can reference the typedef in asm/kvm_host.h without
>>>> needing yet more #ifdefs.
>>>>
>>>> No functional change intended.
>>>>
>>>> Signed-off-by: Sean Christopherson <seanjc@google.com>
>>>> ---
>>>>   arch/x86/include/asm/reboot.h | 2 +-
>>>>   1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/arch/x86/include/asm/reboot.h 
>>>> b/arch/x86/include/asm/reboot.h
>>>> index 6536873f8fc0..d0ef2a678d66 100644
>>>> --- a/arch/x86/include/asm/reboot.h
>>>> +++ b/arch/x86/include/asm/reboot.h
>>>> @@ -25,8 +25,8 @@ void __noreturn machine_real_restart(unsigned int 
>>>> type);
>>>>   #define MRR_BIOS    0
>>>>   #define MRR_APM        1
>>>> -#if IS_ENABLED(CONFIG_KVM_INTEL) || IS_ENABLED(CONFIG_KVM_AMD)
>>>>   typedef void (cpu_emergency_virt_cb)(void);
>>>> +#if IS_ENABLED(CONFIG_KVM_INTEL) || IS_ENABLED(CONFIG_KVM_AMD)
>>>>   void cpu_emergency_register_virt_callback(cpu_emergency_virt_cb 
>>>> *callback);
>>>>   void cpu_emergency_unregister_virt_callback(cpu_emergency_virt_cb 
>>>> *callback);
>>>>   void cpu_emergency_disable_virtualization(void);
>>>
>>> It looks a little it weird.  If other file wants to include
>>> <asm/kvm_host.h> (directly or via <linux/kvm_host.h>) unconditionally 
>>> then
>>> in general I think <asm/kvm_host.h> or <linux/kvm_host.h> should
>>> have something like:
>>>
>>>     #ifdef CONFIG_KVM
>>>
>>>     void func(void);
>>>     ...
>>>
>>>     #else
>>>
>>>     static inline void func(void) {}
>>>
>>>     #endif
>>>
>>> But it seems neither <asm/kvm_host.h> nor <linux/kvm_host.h> has this
>>> pattern.
>>>
>>> I tried to build with !CONFIG_KVM with patch 2 in this series, and I got
>>> below error:
>>
>> Well, yeah.
>>
>>> In file included from ./include/linux/kvm_host.h:45,
>>>                   from arch/x86/events/intel/core.c:17:
>>> ./arch/x86/include/asm/kvm_host.h:1617:9: error: unknown type name
>>> ‘cpu_emergency_virt_cb’
>>>   1617 |         cpu_emergency_virt_cb *emergency_disable;
>>>        |         ^~~~~~~~~~~~~~~~~~~~~
>>>
>>>
>>> Looking at the code, it seems it is because intel_guest_get_msrs() needs
>>> 'struct kvm_pmu' (e.g., it accesses the members of 'struct 
>>> kvm_pmu').  But
>>> it doesn't look the relevant code should be compiled when !CONFIG_KVM.
>>>
>>> So looks a better way is to explicitly use #ifdef CONFIG_KVM around the
>>> relevant code in the arch/x86/events/intel/core.c?
>>
>> Eh, there's no right or wrong way to handle code that is conditionally 
>> compiled.
>> There are always tradeoffs and pros/cons, e.g. the number of #ifdefs, 
>> the amount
>> of effective code validation for all configs, readability, etc.
>>
>> E.g. if there is only one user of a function that conditionally 
>> exists, then
>> having the caller handle the situation might be cleaner.  But if there 
>> are
>> multiple callers, then providing a stub is usually preferable.
> 
> Yeah.
> 
>>
>> IMO, the real problem is that perf pokes into KVM _at all_.  Same for 
>> VFIO.
>> The perf usage is especially egregious, as there is zero reason perf 
>> should need
>> KVM internals[1].  VFIO requires a bit more effort, but I'm fairly 
>> confident that
>> Jason's file-based approach[2] will yield clean, robust code that 
>> minimizes the
>> number of #ifdefs required.
>>
>> I'm planning/hoping to get back to that series in the next few weeks.  
>> As for
>> this small series, I prefer to unconditionally define the typedef, as 
>> it requires
>> no additional #ifdefs, and there are no meaningful downsides to 
>> letting the
>> typedef exist for all kernel builds.
> 
> Seems the final target is to remove those <linux/kvm_host.h> users, or I 
> think a safe-once-for-all solution is to provide the stubs in 
> <linux/kvm_host.h> with:
> 
>      #ifdef CONFIG_KVM
>      ...
>      #else
>      #endif
> 
> In either way, my concerns is it seems modifying the <asm/reboot.h> is a 
> temporary workaround.  And when we reach the final solution I suppose we 
> will need to revert it back to the current way?
> 
> If so, how about manually add a temporary typedef in <asm/kvm_host.h> 
> for now?
> 
>      #ifndef CONFIG_KVM
>      typedef void (cpu_emergency_virt_cb)(void);
>      #endif
> 
> Yes it's ugly, but it's KVM self-contained, and can be removed when ready.
> 
> Anyway, just my 2 cents.
> 

A second thought:

How about we just make all emergency virtualization disable code 
unconditional but not guided by CONFIG_KVM_INTEL || CONFIG_KVM_AMD, 
i.e., revert commit

    261cd5ed934e ("x86/reboot: Expose VMCS crash hooks if and only if 
KVM_{INTEL,AMD} is enabled")

It makes sense anyway from the perspective that it allows the 
out-of-tree kernel module hypervisor to use this mechanism w/o needing 
to have the kernel built with KVM enabled in Kconfig.  Otherwise, 
strictly speaking, IIUC, the kernel won't be able to support out-of-tree 
module hypervisor as there's no other way the module can intercept 
emergency reboot.

This approach avoids the weirdness of the unconditional define for only 
cpu_emergency_virt_cb.