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>?
