Re: [kvm-unit-tests PATCH v3] x86: Add RDTSC test

[Liran Alon <liran.alon@oracle.com>]

> On 2 Dec 2019, at 22:43, Aaron Lewis <aaronlewis@google.com> wrote:
> 
> Verify that the difference between a guest RDTSC instruction and the
> IA32_TIME_STAMP_COUNTER MSR value stored in the VMCS12's VM-exit
> MSR-store list is less than 750 cycles, 99.9% of the time.
> 
> 662f1d1d1931 ("KVM: nVMX: Add support for capturing highest observable L2 TSC”)

Reviewed-by: Liran Alon <liran.alon@oracle.com>

> 
> Signed-off-by: Aaron Lewis <aaronlewis@google.com>
> Reviewed-by: Jim Mattson <jmattson@google.com>
> ---
> x86/vmx.h       |  1 +
> x86/vmx_tests.c | 93 +++++++++++++++++++++++++++++++++++++++++++++++++
> 2 files changed, 94 insertions(+)
> 
> diff --git a/x86/vmx.h b/x86/vmx.h
> index 8496be7..21ba953 100644
> --- a/x86/vmx.h
> +++ b/x86/vmx.h
> @@ -420,6 +420,7 @@ enum Ctrl1 {
> 	CPU_SHADOW_VMCS		= 1ul << 14,
> 	CPU_RDSEED		= 1ul << 16,
> 	CPU_PML                 = 1ul << 17,
> +	CPU_USE_TSC_SCALING	= 1ul << 25,
> };
> 
> enum Intr_type {
> diff --git a/x86/vmx_tests.c b/x86/vmx_tests.c
> index 1d8932f..6ceaf9a 100644
> --- a/x86/vmx_tests.c
> +++ b/x86/vmx_tests.c
> @@ -8790,7 +8790,99 @@ static void vmx_vmcs_shadow_test(void)
> 	enter_guest();
> }
> 
> +/*
> + * This test monitors the difference between a guest RDTSC instruction
> + * and the IA32_TIME_STAMP_COUNTER MSR value stored in the VMCS12
> + * VM-exit MSR-store list when taking a VM-exit on the instruction
> + * following RDTSC.
> + */
> +#define RDTSC_DIFF_ITERS 100000
> +#define RDTSC_DIFF_FAILS 100
> +#define HOST_CAPTURED_GUEST_TSC_DIFF_THRESHOLD 750
> +
> +/*
> + * Set 'use TSC offsetting' and set the guest offset to the
> + * inverse of the host's current TSC value, so that the guest starts running
> + * with an effective TSC value of 0.
> + */
> +static void reset_guest_tsc_to_zero(void)
> +{
> +	TEST_ASSERT_MSG(ctrl_cpu_rev[0].clr & CPU_USE_TSC_OFFSET,
> +			"Expected support for 'use TSC offsetting'");
> +
> +	vmcs_set_bits(CPU_EXEC_CTRL0, CPU_USE_TSC_OFFSET);
> +	vmcs_write(TSC_OFFSET, -rdtsc());
> +}
> +
> +static void rdtsc_vmexit_diff_test_guest(void)
> +{
> +	int i;
> +
> +	for (i = 0; i < RDTSC_DIFF_ITERS; i++)
> +		/* Ensure rdtsc is the last instruction before the vmcall. */
> +		asm volatile("rdtsc; vmcall" : : : "eax", "edx");
> +}
> 
> +/*
> + * This function only considers the "use TSC offsetting" VM-execution
> + * control.  It does not handle "use TSC scaling" (because the latter
> + * isn't available to the host today.)
> + */
> +static unsigned long long host_time_to_guest_time(unsigned long long t)
> +{
> +	TEST_ASSERT(!(ctrl_cpu_rev[0].clr & CPU_SECONDARY) ||
> +		    !(vmcs_read(CPU_EXEC_CTRL1) & CPU_USE_TSC_SCALING));
> +
> +	if (vmcs_read(CPU_EXEC_CTRL0) & CPU_USE_TSC_OFFSET)
> +		t += vmcs_read(TSC_OFFSET);
> +
> +	return t;
> +}
> +
> +static unsigned long long rdtsc_vmexit_diff_test_iteration(void)
> +{
> +	unsigned long long guest_tsc, host_to_guest_tsc;
> +
> +	enter_guest();
> +	skip_exit_vmcall();
> +	guest_tsc = (u32) regs.rax + (regs.rdx << 32);
> +	host_to_guest_tsc = host_time_to_guest_time(exit_msr_store[0].value);
> +
> +	return host_to_guest_tsc - guest_tsc;
> +}
> +
> +static void rdtsc_vmexit_diff_test(void)
> +{
> +	int fail = 0;
> +	int i;
> +
> +	test_set_guest(rdtsc_vmexit_diff_test_guest);
> +
> +	reset_guest_tsc_to_zero();
> +
> +	/*
> +	 * Set up the VMCS12 VM-exit MSR-store list to store just one
> +	 * MSR: IA32_TIME_STAMP_COUNTER. Note that the value stored is
> +	 * in the host time domain (i.e., it is not adjusted according
> +	 * to the TSC multiplier and TSC offset fields in the VMCS12,
> +	 * as a guest RDTSC would be.)
> +	 */
> +	exit_msr_store = alloc_page();
> +	exit_msr_store[0].index = MSR_IA32_TSC;
> +	vmcs_write(EXI_MSR_ST_CNT, 1);
> +	vmcs_write(EXIT_MSR_ST_ADDR, virt_to_phys(exit_msr_store));
> +
> +	for (i = 0; i < RDTSC_DIFF_ITERS; i++) {
> +		if (rdtsc_vmexit_diff_test_iteration() >=
> +		    HOST_CAPTURED_GUEST_TSC_DIFF_THRESHOLD)
> +			fail++;
> +	}
> +
> +	enter_guest();
> +
> +	report("RDTSC to VM-exit delta too high in %d of %d iterations",
> +	       fail < RDTSC_DIFF_FAILS, fail, RDTSC_DIFF_ITERS);
> +}
> 
> static int invalid_msr_init(struct vmcs *vmcs)
> {
> @@ -9056,5 +9148,6 @@ struct vmx_test vmx_tests[] = {
> 	/* Atomic MSR switch tests. */
> 	TEST(atomic_switch_max_msrs_test),
> 	TEST(atomic_switch_overflow_msrs_test),
> +	TEST(rdtsc_vmexit_diff_test),
> 	{ NULL, NULL, NULL, NULL, NULL, {0} },
> };
> -- 
> 2.24.0.393.g34dc348eaf-goog
>