KVM: selftests: Rework OSXSAVE CR4=>CPUID test to play nice with AVX insns

Rework the CR4/CPUID sync test to clear CR4.OSXSAVE, do CPUID, and restore
CR4.OSXSAVE in assembly, so that there is zero chance of AVX instructions
being executed while CR4.OSXSAVE is disabled.  This will allow enabling
CR4.OSXSAVE by default for selftests vCPUs as a general means of playing
nice with AVX instructions.

Reviewed-by: Vitaly Kuznetsov <vkuznets@redhat.com>
Link: https://lore.kernel.org/r/20241003234337.273364-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/tools/testing/selftests/kvm/x86_64/cr4_cpuid_sync_test.c b/tools/testing/selftests/kvm/x86_64/cr4_cpuid_sync_test.c
index 624dc725e14dc0d3170b7fbe6414c92907102027..da818afb7031ebfb93c529039571953580abf86d 100644 (file)
--- a/tools/testing/selftests/kvm/x86_64/cr4_cpuid_sync_test.c
+++ b/tools/testing/selftests/kvm/x86_64/cr4_cpuid_sync_test.c
@@ -19,15 +19,14 @@
 #include "kvm_util.h"
 #include "processor.h"
 
-static inline bool cr4_cpuid_is_sync(void)
-{
-       uint64_t cr4 = get_cr4();
-
-       return (this_cpu_has(X86_FEATURE_OSXSAVE) == !!(cr4 & X86_CR4_OSXSAVE));
-}
+#define MAGIC_HYPERCALL_PORT   0x80
 
 static void guest_code(void)
 {
+       u32 regs[4] = {
+               [KVM_CPUID_EAX] = X86_FEATURE_OSXSAVE.function,
+               [KVM_CPUID_ECX] = X86_FEATURE_OSXSAVE.index,
+       };
        uint64_t cr4;
 
        /* turn on CR4.OSXSAVE */
@@ -36,13 +35,29 @@ static void guest_code(void)
        set_cr4(cr4);
 
        /* verify CR4.OSXSAVE == CPUID.OSXSAVE */
-       GUEST_ASSERT(cr4_cpuid_is_sync());
-
-       /* notify hypervisor to change CR4 */
-       GUEST_SYNC(0);
-
-       /* check again */
-       GUEST_ASSERT(cr4_cpuid_is_sync());
+       GUEST_ASSERT(this_cpu_has(X86_FEATURE_OSXSAVE));
+
+       /*
+        * Notify hypervisor to clear CR4.0SXSAVE, do CPUID and save output,
+        * and then restore CR4.  Do this all in  assembly to ensure no AVX
+        * instructions are executed while OSXSAVE=0.
+        */
+       asm volatile (
+               "out %%al, $" __stringify(MAGIC_HYPERCALL_PORT) "\n\t"
+               "cpuid\n\t"
+               "mov %%rdi, %%cr4\n\t"
+               : "+a" (regs[KVM_CPUID_EAX]),
+                 "=b" (regs[KVM_CPUID_EBX]),
+                 "+c" (regs[KVM_CPUID_ECX]),
+                 "=d" (regs[KVM_CPUID_EDX])
+               : "D" (get_cr4())
+       );
+
+       /* Verify KVM cleared OSXSAVE in CPUID when it was cleared in CR4. */
+       GUEST_ASSERT(!(regs[X86_FEATURE_OSXSAVE.reg] & BIT(X86_FEATURE_OSXSAVE.bit)));
+
+       /* Verify restoring CR4 also restored OSXSAVE in CPUID. */
+       GUEST_ASSERT(this_cpu_has(X86_FEATURE_OSXSAVE));
 
        GUEST_DONE();
 }
@@ -62,13 +77,16 @@ int main(int argc, char *argv[])
                vcpu_run(vcpu);
                TEST_ASSERT_KVM_EXIT_REASON(vcpu, KVM_EXIT_IO);
 
-               switch (get_ucall(vcpu, &uc)) {
-               case UCALL_SYNC:
+               if (vcpu->run->io.port == MAGIC_HYPERCALL_PORT &&
+                   vcpu->run->io.direction == KVM_EXIT_IO_OUT) {
                        /* emulate hypervisor clearing CR4.OSXSAVE */
                        vcpu_sregs_get(vcpu, &sregs);
                        sregs.cr4 &= ~X86_CR4_OSXSAVE;
                        vcpu_sregs_set(vcpu, &sregs);
-                       break;
+                       continue;
+               }
+
+               switch (get_ucall(vcpu, &uc)) {
                case UCALL_ABORT:
                        REPORT_GUEST_ASSERT(uc);
                        break;