[MODERATED] Re: [patch V6 08/14] MDS basics 8

On Fri, Mar 01, 2019 at 10:47:46PM +0100, speck for Thomas Gleixner wrote:
> CPUs which are affected by L1TF and MDS mitigate MDS with the L1D Flush on
> VMENTER when updated microcode is installed.
> 
> If a CPU is not affected by L1TF or if the L1D Flush is not in use, then
> MDS mitigation needs to be invoked explicit.
> 
> For these cases, follow the host mitigation state and invoke the MDS
> mitigation before VMENTER.
> 
> Signed-off-by: Thomas Gleixner <[email protected]>
> Reviewed-by: Greg Kroah-Hartman <[email protected]>
> ---
> V4 --> V5: Fix changelog
> ---
>  arch/x86/kernel/cpu/bugs.c |    1 +
>  arch/x86/kvm/vmx/vmx.c     |    2 ++
>  2 files changed, 3 insertions(+)
> 
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -65,6 +65,7 @@ DEFINE_STATIC_KEY_FALSE(switch_mm_always
>  
>  /* Control MDS CPU buffer clear before returning to user space */
>  DEFINE_STATIC_KEY_FALSE(mds_user_clear);
> +EXPORT_SYMBOL_GPL(mds_user_clear);
>  
>  void __init check_bugs(void)
>  {
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6371,6 +6371,8 @@ static void __vmx_vcpu_run(struct kvm_vc

We may want to add a comment below to summarize what's explained
in the changelog. git blame tends to lose prime history after any
future most unsignificant variable rename. Something like:

  +     /* l1tf mitigation, if present, spares us mds mitigation */

>  	if (static_branch_unlikely(&vmx_l1d_should_flush))
>  		vmx_l1d_flush(vcpu);
> +	else if (static_branch_unlikely(&mds_user_clear))
> +		mds_clear_cpu_buffers();

Reviewed-by: Frederic Weisbecker <[email protected]>

Thanks.