Re: [PATCH 08/14] x86/entry: Optimize local_db_save() for virt

[Sean Christopherson]

On Fri, May 29, 2020 at 11:27:36PM +0200, Peter Zijlstra wrote:
> Because DRn access is 'difficult' with virt; but the DR7 read is
> cheaper than a cacheline miss on native, add a virt specific
> fast path to local_db_save(), such that when breakpoints are not in
> use we avoid touching DRn entirely.
> 
> Suggested-by: Andy Lutomirski <luto@xxxxxxxxxx>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/debugreg.h |    7 ++++++-
>  arch/x86/kernel/hw_breakpoint.c |   26 ++++++++++++++++++++++----
>  arch/x86/kvm/vmx/nested.c       |    2 +-
>  3 files changed, 29 insertions(+), 6 deletions(-)

...

> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -3028,9 +3028,9 @@ static int nested_vmx_check_vmentry_hw(s
>  	/*
>  	 * VMExit clears RFLAGS.IF and DR7, even on a consistency check.
>  	 */
> -	local_irq_enable();
>  	if (hw_breakpoint_active())
>  		set_debugreg(__this_cpu_read(cpu_dr7), 7);
> +	local_irq_enable();
>  	preempt_enable();

This should be a separate patch, probably with:

  Cc: stable@xxxxxxxxxxxxxxx
  Fixes: 52017608da33 ("KVM: nVMX: add option to perform early consistency checks via H/W")