Re: [PATCH v2 2/2] KVM: selftests: Verify that L2's LMSW leaves L1-owned CR0 bits alone

[Sean Christopherson <seanjc@google.com>]

On Thu, Oct 29, 2026, Mahmoud Abumandour wrote:
> Verify that an LMSW in L2 that exits to L0 only leaves an L1-owned CR0.TS
> set, and that L2 reads an L1-owned CR0.NE from the read shadow.
> 
> Assisted-by: LLM
> Signed-off-by: Mahmoud Abumandour <ma.mandourr@gmail.com>
> ---
>  .../kvm/x86/nested_exceptions_test.c          | 36 +++++++++++++++++++
>  1 file changed, 36 insertions(+)
> 
> diff --git a/tools/testing/selftests/kvm/x86/nested_exceptions_test.c b/tools/testing/selftests/kvm/x86/nested_exceptions_test.c
> index f47432526e..e33d2e0db0 100644
> --- a/tools/testing/selftests/kvm/x86/nested_exceptions_test.c
> +++ b/tools/testing/selftests/kvm/x86/nested_exceptions_test.c
> @@ -109,6 +109,40 @@ static void l1_svm_code(struct svm_test_data *svm)
>  	GUEST_DONE();
>  }
>  
> +static u64 l2_cr0;
> +
> +static void l2_lmsw_test(void)
> +{
> +	/* Toggle CR0.MP, which KVM intercepts and L1 doesn't. */
> +	asm volatile("lmsw %w0" : : "r" (get_cr0() ^ X86_CR0_MP));

We should add lmsw(), and also test MOV CR0 because it's easy to do so.

> +	l2_cr0 = get_cr0();
> +	asm volatile("fnop" ::: "memory");
> +	GUEST_FAIL("L2's FNOP didn't take #NM");

*sigh*

Again, I have no idea if you/AI even realize the careful dance that's going on
here.  I *wanted* to say this should simply have L2 assert on the expected CR0
value, but doing so results in false passes because trying to report the assert
means making a ucall(), ucall_alloc() does a memset(), and the compiler likes to
generate SIMD instructions for memset().  I.e. trying to report a failure triggers
the #NM that L1 expects.

So the code is fine, but this needs a big fat pile of comments and the changelog
needs to explain what's going on as well.

> +}
> +
> +/*
> + * L1 owns CR0.TS (set) and CR0.NE, both with a read shadow of 0.  L2's LMSW
> + * must leave both alone: FNOP takes #NM and L2 reads both as 0.
> + */
> +static void vmx_run_l2_lmsw(void)
> +{
> +	u64 cr0 = vmread(GUEST_CR0);
> +
> +	vmwrite(CR0_GUEST_HOST_MASK, X86_CR0_TS | X86_CR0_NE);
> +	vmwrite(CR0_READ_SHADOW, cr0 & ~(X86_CR0_TS | X86_CR0_NE));
> +	vmwrite(GUEST_CR0, cr0 | X86_CR0_TS);
> +	vmwrite(EXCEPTION_BITMAP, BIT(NM_VECTOR));
> +	vmwrite(GUEST_RIP, (u64)l2_lmsw_test);
> +	vmresume();
> +
> +	GUEST_ASSERT_EQ(vmread(VM_EXIT_REASON), EXIT_REASON_EXCEPTION_NMI);
> +	GUEST_ASSERT_EQ(vmread(VM_EXIT_INTR_INFO) & 0xff, NM_VECTOR);
> +	GUEST_ASSERT_EQ(l2_cr0, (cr0 & ~(X86_CR0_TS | X86_CR0_NE)) ^ X86_CR0_MP);

Just use vmx_run_l2(), most of this is copy+pasted from there anyways.

> +
> +	vmwrite(CR0_GUEST_HOST_MASK, 0);
> +	vmwrite(GUEST_CR0, cr0);
> +}
> +
>  static void vmx_run_l2(void *l2_code, int vector, u32 error_code)
>  {
>  	vmwrite(GUEST_RIP, (u64)l2_code);
> @@ -149,6 +183,8 @@ static void l1_vmx_code(struct vmx_pages *vmx)
>  	vmwrite(EXCEPTION_BITMAP, INTERCEPT_SS_DF);
>  	vmx_run_l2(l2_ss_injected_df_test, DF_VECTOR, DF_ERROR_CODE);
>  
> +	vmx_run_l2_lmsw();
> +
>  	vmwrite(EXCEPTION_BITMAP, INTERCEPT_SS);
>  	vmx_run_l2(l2_ss_injected_tf_test, FAKE_TRIPLE_FAULT_VECTOR, 0);
>  	GUEST_ASSERT_EQ(vmread(VM_EXIT_REASON), EXIT_REASON_TRIPLE_FAULT);
> -- 
> 2.25.1
> 
>