Re: [PATCH] icount: don't adjust virtual time backwards after warp

[Paolo Bonzini <pbonzini@redhat.com>]

On 6/27/23 08:14, Nicholas Piggin wrote:
> The icount-based QEMU_CLOCK_VIRTUAL runs ahead of the RT clock at times.
> When warping, it is possible it is still ahead at the end of the warp,
> which causes icount adaptive mode to adjust it backward. This can result
> in the machine observing time going backwards.
> 
> Prevent this by clamping adaptive adjustment to 0 at minimum.

Queued, thanks.

Paolo

> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>   softmmu/icount.c | 9 +++++++--
>   1 file changed, 7 insertions(+), 2 deletions(-)
> 
> diff --git a/softmmu/icount.c b/softmmu/icount.c
> index 4504433e16..486ea7ef41 100644
> --- a/softmmu/icount.c
> +++ b/softmmu/icount.c
> @@ -259,11 +259,16 @@ static void icount_warp_rt(void)
>           warp_delta = clock - timers_state.vm_clock_warp_start;
>           if (icount_enabled() == 2) {
>               /*
> -             * In adaptive mode, do not let QEMU_CLOCK_VIRTUAL run too
> -             * far ahead of real time.
> +             * In adaptive mode, do not let QEMU_CLOCK_VIRTUAL run too far
> +             * ahead of real time (it might already be ahead so careful not
> +             * to go backwards).
>                */
>               int64_t cur_icount = icount_get_locked();
>               int64_t delta = clock - cur_icount;
> +
> +            if (delta < 0) {
> +                delta = 0;
> +            }
>               warp_delta = MIN(warp_delta, delta);
>           }
>           qatomic_set_i64(&timers_state.qemu_icount_bias,