target/i386/kvm/kvm.c | 17 +++++++++++++---- 1 file changed, 13 insertions(+), 4 deletions(-)
KVM support for AMX includes a new system attribute, KVM_X86_XCOMP_GUEST_SUPP.
Commit 19db68ca68 ("x86: Grant AMX permission for guest", 2022-03-15) however
did not fully consider the behavior on older kernels. First, it warns
too aggressively. Second, it invokes the KVM_GET_DEVICE_ATTR ioctl
unconditionally and then uses the "bitmask" variable, which remains
uninitialized if the ioctl fails.
While at it, explain why the ioctl is needed and KVM_GET_SUPPORTED_CPUID
is not enough.
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---
target/i386/kvm/kvm.c | 17 +++++++++++++----
1 file changed, 13 insertions(+), 4 deletions(-)
diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
index ce0e8a4042..f2c9f7b5ca 100644
--- a/target/i386/kvm/kvm.c
+++ b/target/i386/kvm/kvm.c
@@ -412,6 +412,12 @@ uint32_t kvm_arch_get_supported_cpuid(KVMState *s, uint32_t function,
}
} else if (function == 0xd && index == 0 &&
(reg == R_EAX || reg == R_EDX)) {
+ /*
+ * The value returned by KVM_GET_SUPPORTED_CPUID does not include
+ * features that still have to be enabled with the arch_prctl
+ * system call. QEMU needs the full value, which is retrieved
+ * with KVM_GET_DEVICE_ATTR.
+ */
struct kvm_device_attr attr = {
.group = 0,
.attr = KVM_X86_XCOMP_GUEST_SUPP,
@@ -420,13 +426,16 @@ uint32_t kvm_arch_get_supported_cpuid(KVMState *s, uint32_t function,
bool sys_attr = kvm_check_extension(s, KVM_CAP_SYS_ATTRIBUTES);
if (!sys_attr) {
- warn_report("cannot get sys attribute capabilities %d", sys_attr);
+ return ret;
}
int rc = kvm_ioctl(s, KVM_GET_DEVICE_ATTR, &attr);
- if (rc == -1 && (errno == ENXIO || errno == EINVAL)) {
- warn_report("KVM_GET_DEVICE_ATTR(0, KVM_X86_XCOMP_GUEST_SUPP) "
- "error: %d", rc);
+ if (rc == -1) {
+ if (errno != ENXIO) {
+ warn_report("KVM_GET_DEVICE_ATTR(0, KVM_X86_XCOMP_GUEST_SUPP) "
+ "error: %d", rc);
+ }
+ return ret;
}
ret = (reg == R_EAX) ? bitmask : bitmask >> 32;
} else if (function == 0x80000001 && reg == R_ECX) {
--
2.35.1
Am 18.03.22 um 16:26 schrieb Paolo Bonzini: > KVM support for AMX includes a new system attribute, KVM_X86_XCOMP_GUEST_SUPP. > Commit 19db68ca68 ("x86: Grant AMX permission for guest", 2022-03-15) however > did not fully consider the behavior on older kernels. First, it warns > too aggressively. Second, it invokes the KVM_GET_DEVICE_ATTR ioctl > unconditionally and then uses the "bitmask" variable, which remains > uninitialized if the ioctl fails. > > While at it, explain why the ioctl is needed and KVM_GET_SUPPORTED_CPUID > is not enough. > > Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> > --- > target/i386/kvm/kvm.c | 17 +++++++++++++---- > 1 file changed, 13 insertions(+), 4 deletions(-) > > diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c > index ce0e8a4042..f2c9f7b5ca 100644 > --- a/target/i386/kvm/kvm.c > +++ b/target/i386/kvm/kvm.c > @@ -412,6 +412,12 @@ uint32_t kvm_arch_get_supported_cpuid(KVMState *s, uint32_t function, > } > } else if (function == 0xd && index == 0 && > (reg == R_EAX || reg == R_EDX)) { > + /* > + * The value returned by KVM_GET_SUPPORTED_CPUID does not include > + * features that still have to be enabled with the arch_prctl > + * system call. QEMU needs the full value, which is retrieved > + * with KVM_GET_DEVICE_ATTR. > + */ > struct kvm_device_attr attr = { > .group = 0, > .attr = KVM_X86_XCOMP_GUEST_SUPP, > @@ -420,13 +426,16 @@ uint32_t kvm_arch_get_supported_cpuid(KVMState *s, uint32_t function, > > bool sys_attr = kvm_check_extension(s, KVM_CAP_SYS_ATTRIBUTES); > if (!sys_attr) { > - warn_report("cannot get sys attribute capabilities %d", sys_attr); > + return ret; > } > > int rc = kvm_ioctl(s, KVM_GET_DEVICE_ATTR, &attr); > - if (rc == -1 && (errno == ENXIO || errno == EINVAL)) { > - warn_report("KVM_GET_DEVICE_ATTR(0, KVM_X86_XCOMP_GUEST_SUPP) " > - "error: %d", rc); > + if (rc == -1) { Hi Paolo, this is kvm_ioctl() not ioctl(). kvm_ioctl() returns -errno on error. With best regards, Volker > + if (errno != ENXIO) { > + warn_report("KVM_GET_DEVICE_ATTR(0, KVM_X86_XCOMP_GUEST_SUPP) " > + "error: %d", rc); > + } > + return ret; > } > ret = (reg == R_EAX) ? bitmask : bitmask >> 32; > } else if (function == 0x80000001 && reg == R_ECX) {
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