Re: [PATCH] x86/hvm: Fix fast singlestep state persistence

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 08/02/2024 9:20 pm, Petr Beneš wrote:
> From: Petr Beneš <w1benny@xxxxxxxxx>
>
> This patch addresses an issue where the fast singlestep setting would persist
> despite xc_domain_debug_control being called with 
> XEN_DOMCTL_DEBUG_OP_SINGLE_STEP_OFF.
> Specifically, if fast singlestep was enabled in a VMI session and that session
> stopped before the MTF trap occurred, the fast singlestep setting remained
> active even though MTF itself was disabled.  This led to a situation where, 
> upon
> starting a new VMI session, the first event to trigger an EPT violation would
> cause the corresponding EPT event callback to be skipped due to the lingering
> fast singlestep setting.
>
> The fix ensures that the fast singlestep setting is properly reset when
> disabling single step debugging operations.
>
> Signed-off-by: Petr Beneš <w1benny@xxxxxxxxx>
> ---
>  xen/arch/x86/hvm/hvm.c | 32 +++++++++++++++++++++++---------
>  1 file changed, 23 insertions(+), 9 deletions(-)
>
> diff --git a/xen/arch/x86/hvm/hvm.c b/xen/arch/x86/hvm/hvm.c
> index e8deeb0222..4f988de4c1 100644
> --- a/xen/arch/x86/hvm/hvm.c
> +++ b/xen/arch/x86/hvm/hvm.c
> @@ -5160,26 +5160,40 @@ long do_hvm_op(unsigned long op, 
> XEN_GUEST_HANDLE_PARAM(void) arg)
>  
>  int hvm_debug_op(struct vcpu *v, int32_t op)
>  {
> -    int rc;
> +    int rc = 0;
>  
>      switch ( op )
>      {
>          case XEN_DOMCTL_DEBUG_OP_SINGLE_STEP_ON:
>          case XEN_DOMCTL_DEBUG_OP_SINGLE_STEP_OFF:
> -            rc = -EOPNOTSUPP;
>              if ( !cpu_has_monitor_trap_flag )
> -                break;
> -            rc = 0;
> -            vcpu_pause(v);
> -            v->arch.hvm.single_step =
> -                (op == XEN_DOMCTL_DEBUG_OP_SINGLE_STEP_ON);
> -            vcpu_unpause(v); /* guest will latch new state */
> +                return -EOPNOTSUPP;
>              break;
>          default:
> -            rc = -ENOSYS;
> +            return -ENOSYS;
> +    }
> +
> +    vcpu_pause(v);
> +
> +    switch ( op )
> +    {
> +        case XEN_DOMCTL_DEBUG_OP_SINGLE_STEP_ON:
> +            v->arch.hvm.single_step = true;
> +            break;
> +
> +        case XEN_DOMCTL_DEBUG_OP_SINGLE_STEP_OFF:
> +            v->arch.hvm.single_step = false;
> +            v->arch.hvm.fast_single_step.enabled = false;
> +            v->arch.hvm.fast_single_step.p2midx = 0;
>              break;
> +
> +        default:
> +            ASSERT_UNREACHABLE();

Two things.

First, this reads as if it's reachable, and therefore wrong.  You
probably want an /* Excluded above */ comment to point out why it's safe
in this case.

Second, I know you're copying the existing switch(), but it wasn't
compliant with Xen's coding style.  The cases and their clauses should
have one fewer indentation level.

I'm happy to fix up both on commit.

~Andrew