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

[Sean Christopherson <seanjc@google.com>]

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.

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.

[1] https://lore.kernel.org/all/20230916003118.2540661-21-seanjc@google.com
[2] https://lore.kernel.org/all/ZXkVSKULLivrMkBl@google.com

> And it seems vfio does it in vfio_main.c:
> 
> 	#if IS_ENABLED(CONFIG_KVM)
> 	#include <linux/kvm_host.h>
> 	#endif
> 
> 	#if IS_ENABLED(CONFIG_KVM)
> 	void vfio_device_get_kvm_safe(struct vfio_device *device, 
> 			struct kvm *kvm)
> 	{
> 		...
> 	}
> 	...
> 	#endif
> 
> 
> The only remaining weird thing is 'struct kvm *kvm' is still used
> unconditionally in vfio_main.c, but I think the reason it builds fine with
> !CONFIG_KVM is because <linux/vfio.h> declares it explicitly:
> 
> 	struct kvm;
> 	struct iommufd_ctx;
> 	...
> 
> So it seems to me that this patch around 'cpu_emergency_virt_cb' is more
> like a workaround of existing non-perfect <linux/kvm_host.h> and/or
> <asm/kvm_host.h>?