Re: [PATCH v12 45/46] KVM: selftests: hyperv_svm_test: Introduce L2 TLB flush test

[Sean Christopherson <seanjc@google.com>]

On Fri, Oct 21, 2022, Vitaly Kuznetsov wrote:
> Enable Hyper-V L2 TLB flush and check that Hyper-V TLB flush hypercalls
> from L2 don't exit to L1 unless 'TlbLockCount' is set in the Partition
> assist page.
> 
> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> ---
>  .../selftests/kvm/include/x86_64/svm.h        |  4 ++
>  .../selftests/kvm/x86_64/hyperv_svm_test.c    | 61 +++++++++++++++++--
>  2 files changed, 60 insertions(+), 5 deletions(-)
> 
> diff --git a/tools/testing/selftests/kvm/include/x86_64/svm.h b/tools/testing/selftests/kvm/include/x86_64/svm.h
> index 483e6ae12f69..4803e1056055 100644
> --- a/tools/testing/selftests/kvm/include/x86_64/svm.h
> +++ b/tools/testing/selftests/kvm/include/x86_64/svm.h
> @@ -76,6 +76,10 @@ struct hv_vmcb_enlightenments {
>   */
>  #define HV_VMCB_NESTED_ENLIGHTENMENTS (1U << 31)
>  
> +/* Synthetic VM-Exit */
> +#define HV_SVM_EXITCODE_ENL			0xf0000000
> +#define HV_SVM_ENL_EXITCODE_TRAP_AFTER_FLUSH	(1)
> +
>  struct __attribute__ ((__packed__)) vmcb_control_area {
>  	u32 intercept_cr;
>  	u32 intercept_dr;
> diff --git a/tools/testing/selftests/kvm/x86_64/hyperv_svm_test.c b/tools/testing/selftests/kvm/x86_64/hyperv_svm_test.c
> index 1c3fc38b4f15..edb779615a79 100644
> --- a/tools/testing/selftests/kvm/x86_64/hyperv_svm_test.c
> +++ b/tools/testing/selftests/kvm/x86_64/hyperv_svm_test.c
> @@ -25,6 +25,8 @@
>  
>  void l2_guest_code(void)
>  {
> +	u64 unused;
> +
>  	GUEST_SYNC(3);
>  	/* Exit to L1 */
>  	vmmcall();
> @@ -38,11 +40,30 @@ void l2_guest_code(void)
>  
>  	GUEST_SYNC(5);
>  
> +	/* L2 TLB flush tests */
> +	hyperv_hypercall(HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE |
> +			 HV_HYPERCALL_FAST_BIT, 0x0,
> +			 HV_FLUSH_ALL_VIRTUAL_ADDRESS_SPACES |
> +			 HV_FLUSH_ALL_PROCESSORS);
> +	rdmsr(MSR_FS_BASE);

Why doesn't the SVM test need to clobber GPRs?

> +	/*
> +	 * Note: hypercall status (RAX) is not preserved correctly by L1 after
> +	 * synthetic vmexit, use unchecked version.
> +	 */
> +	__hyperv_hypercall(HVCALL_FLUSH_VIRTUAL_ADDRESS_SPACE |
> +			   HV_HYPERCALL_FAST_BIT, 0x0,
> +			   HV_FLUSH_ALL_VIRTUAL_ADDRESS_SPACES |
> +			   HV_FLUSH_ALL_PROCESSORS, &unused);
> +	/* Make sure we're not issuing Hyper-V TLB flush call again */
> +	__asm__ __volatile__ ("mov $0xdeadbeef, %rcx");

Heh, and a patch to vmmcall() itself "safe"...

> +
>  	/* Done, exit to L1 and never come back.  */
>  	vmmcall();
>  }