KVM: selftests: Configure XCR0 to max supported value by default

To play nice with compilers generating AVX instructions, set CR4.OSXSAVE
and configure XCR0 by default when creating selftests vCPUs.  Some distros
have switched gcc to '-march=x86-64-v3' by default, and while it's hard to
find a CPU which doesn't support AVX today, many KVM selftests fail with

  ==== Test Assertion Failure ====
    lib/x86_64/processor.c:570: Unhandled exception in guest
    pid=72747 tid=72747 errno=4 - Interrupted system call
    Unhandled exception '0x6' at guest RIP '0x4104f7'

due to selftests not enabling AVX by default for the guest.  The failure
is easy to reproduce elsewhere with:

   $ make clean && CFLAGS='-march=x86-64-v3' make -j && ./x86_64/kvm_pv_test

E.g. gcc-13 with -march=x86-64-v3 compiles this chunk from selftests'
kvm_fixup_exception():

        regs->rip = regs->r11;
        regs->r9 = regs->vector;
        regs->r10 = regs->error_code;

into this monstronsity (which is clever, but oof):

  405313:       c4 e1 f9 6e c8          vmovq  %rax,%xmm1
  405318:       48 89 68 08             mov    %rbp,0x8(%rax)
  40531c:       48 89 e8                mov    %rbp,%rax
  40531f:       c4 c3 f1 22 c4 01       vpinsrq $0x1,%r12,%xmm1,%xmm0
  405325:       49 89 6d 38             mov    %rbp,0x38(%r13)
  405329:       c5 fa 7f 45 00          vmovdqu %xmm0,0x0(%rbp)

Alternatively, KVM selftests could explicitly restrict the compiler to
-march=x86-64-v2, but odds are very good that punting on AVX enabling will
simply result in tests that "need" AVX doing their own thing, e.g. there
are already three or so additional cleanups that can be done on top.

Reported-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Closes: https://lore.kernel.org/all/20240920154422.2890096-1-vkuznets@redhat.com
Reviewed-and-tested-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20241003234337.273364-6-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

tools/testing/selftests/kvm/include/x86_64/processor.h

@@ -1049,6 +1049,11 @@ static inline void vcpu_set_cpuid(struct kvm_vcpu *vcpu)
 	vcpu_ioctl(vcpu, KVM_GET_CPUID2, vcpu->cpuid);
 }
 
+static inline void vcpu_get_cpuid(struct kvm_vcpu *vcpu)
+{
+	vcpu_ioctl(vcpu, KVM_GET_CPUID2, vcpu->cpuid);
+}
+
 void vcpu_set_cpuid_property(struct kvm_vcpu *vcpu,
 			     struct kvm_x86_cpu_property property,
 			     uint32_t value);

tools/testing/selftests/kvm/lib/x86_64/processor.c

@@ -506,6 +506,8 @@ static void vcpu_init_sregs(struct kvm_vm *vm, struct kvm_vcpu *vcpu)
 
 	sregs.cr0 = X86_CR0_PE | X86_CR0_NE | X86_CR0_PG;
 	sregs.cr4 |= X86_CR4_PAE | X86_CR4_OSFXSR;
+	if (kvm_cpu_has(X86_FEATURE_XSAVE))
+		sregs.cr4 |= X86_CR4_OSXSAVE;
 	sregs.efer |= (EFER_LME | EFER_LMA | EFER_NX);
 
 	kvm_seg_set_unusable(&sregs.ldt);
@@ -519,6 +521,20 @@ static void vcpu_init_sregs(struct kvm_vm *vm, struct kvm_vcpu *vcpu)
 	vcpu_sregs_set(vcpu, &sregs);
 }
 
+static void vcpu_init_xcrs(struct kvm_vm *vm, struct kvm_vcpu *vcpu)
+{
+	struct kvm_xcrs xcrs = {
+		.nr_xcrs = 1,
+		.xcrs[0].xcr = 0,
+		.xcrs[0].value = kvm_cpu_supported_xcr0(),
+	};
+
+	if (!kvm_cpu_has(X86_FEATURE_XSAVE))
+		return;
+
+	vcpu_xcrs_set(vcpu, &xcrs);
+}
+
 static void set_idt_entry(struct kvm_vm *vm, int vector, unsigned long addr,
 			  int dpl, unsigned short selector)
 {
@@ -675,6 +691,7 @@ struct kvm_vcpu *vm_arch_vcpu_add(struct kvm_vm *vm, uint32_t vcpu_id)
 	vcpu = __vm_vcpu_add(vm, vcpu_id);
 	vcpu_init_cpuid(vcpu, kvm_get_supported_cpuid());
 	vcpu_init_sregs(vm, vcpu);
+	vcpu_init_xcrs(vm, vcpu);
 
 	/* Setup guest general purpose registers */
 	vcpu_regs_get(vcpu, &regs);
@@ -686,6 +703,13 @@ struct kvm_vcpu *vm_arch_vcpu_add(struct kvm_vm *vm, uint32_t vcpu_id)
 	mp_state.mp_state = 0;
 	vcpu_mp_state_set(vcpu, &mp_state);
 
+	/*
+	 * Refresh CPUID after setting SREGS and XCR0, so that KVM's "runtime"
+	 * updates to guest CPUID, e.g. for OSXSAVE and XSAVE state size, are
+	 * reflected into selftests' vCPU CPUID cache, i.e. so that the cache
+	 * is consistent with vCPU state.
+	 */
+	vcpu_get_cpuid(vcpu);
 	return vcpu;
 }
 

tools/testing/selftests/kvm/x86_64/xcr0_cpuid_test.c

@@ -48,16 +48,16 @@ do {									\
 
 static void guest_code(void)
 {
-	uint64_t xcr0_reset;
+	uint64_t initial_xcr0;
 	uint64_t supported_xcr0;
 	int i, vector;
 
 	set_cr4(get_cr4() | X86_CR4_OSXSAVE);
 
-	xcr0_reset = xgetbv(0);
+	initial_xcr0 = xgetbv(0);
 	supported_xcr0 = this_cpu_supported_xcr0();
 
-	GUEST_ASSERT(xcr0_reset == XFEATURE_MASK_FP);
+	GUEST_ASSERT(initial_xcr0 == supported_xcr0);
 
 	/* Check AVX */
 	ASSERT_XFEATURE_DEPENDENCIES(supported_xcr0,