[PATCH 5.15 500/917] KVM: selftests: Fix nested SVM tests when built with clang

Greg Kroah-Hartman
2021-11-15T23:41:59Z

From: Jim Mattson <jmattson@google.com>

[ Upstream commit ed290e1c20da19fa100a3e0f421aa31b65984960 ]

Though gcc conveniently compiles a simple memset to "rep stos," clang
prefers to call the libc version of memset. If a test is dynamically
linked, the libc memset isn't available in L1 (nor is the PLT or the
GOT, for that matter). Even if the test is statically linked, the libc
memset may choose to use some CPU features, like AVX, which may not be
enabled in L1. Note that __builtin_memset doesn't solve the problem,
because (a) the compiler is free to call memset anyway, and (b)
__builtin_memset may also choose to use features like AVX, which may
not be available in L1.

To avoid a myriad of problems, use an explicit "rep stos" to clear the
VMCB in generic_svm_setup(), which is called both from L0 and L1.

Reported-by: Ricardo Koller <ricarkol@google.com>
Signed-off-by: Jim Mattson <jmattson@google.com>
Fixes: 20ba262f8631a ("selftests: KVM: AMD Nested test infrastructure")
Message-Id: <20210930003649.4026553-1-jmattson@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
tools/testing/selftests/kvm/lib/x86_64/svm.c | 14 +++++++++++++-
1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/tools/testing/selftests/kvm/lib/x86_64/svm.c b/tools/testing/selftests/kvm/lib/x86_64/svm.c
index 2ac98d70d02bd..161eba7cd1289 100644
--- a/tools/testing/selftests/kvm/lib/x86_64/svm.c
+++ b/tools/testing/selftests/kvm/lib/x86_64/svm.c
@@ -54,6 +54,18 @@ static void vmcb_set_seg(struct vmcb_seg *seg, u16 selector,
seg->base = base;
}

+/*
+ * Avoid using memset to clear the vmcb, since libc may not be
+ * available in L1 (and, even if it is, features that libc memset may
+ * want to use, like AVX, may not be enabled).
+ */
+static void clear_vmcb(struct vmcb *vmcb)
+{
+ int n = sizeof(*vmcb) / sizeof(u32);
+
+ asm volatile ("rep stosl" : "+c"(n), "+D"(vmcb) : "a"(0) : "memory");
+}
+
void generic_svm_setup(struct svm_test_data *svm, void *guest_rip, void *guest_rsp)
{
struct vmcb *vmcb = svm->vmcb;
@@ -70,7 +82,7 @@ void generic_svm_setup(struct svm_test_data *svm, void *guest_rip, void *guest_r
wrmsr(MSR_EFER, efer | EFER_SVME);
wrmsr(MSR_VM_HSAVE_PA, svm->save_area_gpa);

- memset(vmcb, 0, sizeof(*vmcb));
+ clear_vmcb(vmcb);
asm volatile ("vmsave %0\n\t" : : "a" (vmcb_gpa) : "memory");
vmcb_set_seg(&save->es, get_es(), 0, -1U, data_seg_attr);
vmcb_set_seg(&save->cs, get_cs(), 0, -1U, code_seg_attr);
--
2.33.0