Re: [PATCH] KVM: selftests: Fix nested SVM tests when built with clang

[Ricardo Koller <ricarkol@google.com>]

On Wed, Sep 29, 2021 at 05:36:49PM -0700, Jim Mattson wrote:
> 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")
> ---
>  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 2ac98d70d02b..161eba7cd128 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);
> -- 
> 2.33.0.685.g46640cef36-goog
> 

Reviewed-by: Ricardo Koller <ricarkol@google.com>