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@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.
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.
