On 14/08/2024 5:50 am, Isaku Yamahata wrote:
On Tue, Aug 13, 2024 at 01:12:55PM +0800,
Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx> wrote:
Check whether a KVM hypercall needs to exit to userspace or not based on
hypercall_exit_enabled field of struct kvm_arch.
Userspace can request a hypercall to exit to userspace for handling by
enable KVM_CAP_EXIT_HYPERCALL and the enabled hypercall will be set in
hypercall_exit_enabled. Make the check code generic based on it.
Signed-off-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>
---
arch/x86/kvm/x86.c | 4 ++--
arch/x86/kvm/x86.h | 7 +++++++
2 files changed, 9 insertions(+), 2 deletions(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index af6c8cf6a37a..6e16c9751af7 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -10226,8 +10226,8 @@ int kvm_emulate_hypercall(struct kvm_vcpu *vcpu)
cpl = kvm_x86_call(get_cpl)(vcpu);
ret = __kvm_emulate_hypercall(vcpu, nr, a0, a1, a2, a3, op_64_bit, cpl);
- if (nr == KVM_HC_MAP_GPA_RANGE && !ret)
- /* MAP_GPA tosses the request to the user space. */
+ if (!ret && is_kvm_hc_exit_enabled(vcpu->kvm, nr))
+ /* The hypercall is requested to exit to userspace. */
return 0;
if (!op_64_bit)
diff --git a/arch/x86/kvm/x86.h b/arch/x86/kvm/x86.h
index 50596f6f8320..0cbec76b42e6 100644
--- a/arch/x86/kvm/x86.h
+++ b/arch/x86/kvm/x86.h
@@ -547,4 +547,11 @@ int kvm_sev_es_string_io(struct kvm_vcpu *vcpu, unsigned int size,
unsigned int port, void *data, unsigned int count,
int in);
+static inline bool is_kvm_hc_exit_enabled(struct kvm *kvm, unsigned long hc_nr)
+{
+ if(WARN_ON_ONCE(hc_nr >= sizeof(kvm->arch.hypercall_exit_enabled) * 8))
+ return false;
Is this to detect potential bug? Maybe
BUILD_BUG_ON(__builtin_constant_p(hc_nr) &&
!(BIT(hc_nr) & KVM_EXIT_HYPERCALL_VALID_MASK));
Overkill?
I don't think this is the correct way to use __builtin_constant_p(),
i.e. it doesn't make sense to use __builtin_constant_p() in BUILD_BUG_ON().
IIUC you need some build time guarantee here, but __builtin_constant_p()
can return false, in which case the above BUILD_BUG_ON() does nothing,
which defeats the purpose.
On the other hand, albeit WARN_ON_ONCE() is runtime check, but it is
always there.
In fact, the @hc_nr (or @nr) in the kvm_emulate_hypercall() is read from
the RAX register:
nr = kvm_rax_read(vcpu);
So I don't see how the compiler can be smart enough to determine the
value at compile time.
In fact, I tried to build by removing the __builtin_constant_p() but got
below error (sorry for text wrap but you can see the error I believe).
root@server:/home/kai/tdx/linux# make M=arch/x86/kvm/ W=1
CC [M] arch/x86/kvm/x86.o
In file included from <command-line>:
In function ‘is_kvm_hc_exit_enabled’,
inlined from ‘kvm_emulate_hypercall’ at arch/x86/kvm/x86.c:10254:14:
././include/linux/compiler_types.h:510:45: error: call to
‘__compiletime_assert_3873’ declared with attribute error: BUILD_BUG_ON
failed: !(BIT(hc_nr) & KVM_EXIT_HYPERCALL_VALID_MASK)
510 | _compiletime_assert(condition, msg,
__compiletime_assert_, __COUNTER__)
| ^
