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

[Huang, Kai]

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@xxxxxxxxxx>
> > > ---
> > >   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.




> [1] https://lore.kernel.org/all/20230916003118.2540661-21-seanjc@xxxxxxxxxx
> [2] https://lore.kernel.org/all/ZXkVSKULLivrMkBl@xxxxxxxxxx
>
> > 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>?