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

* [PATCH RFC] icount: don't adjust virtual time backwards after warp
@ 2023-06-25  8:37 Nicholas Piggin
  2023-06-26 11:08 ` Paolo Bonzini
  0 siblings, 1 reply; 4+ messages in thread
From: Nicholas Piggin @ 2023-06-25  8:37 UTC (permalink / raw)
  To: Paolo Bonzini; +Cc: Nicholas Piggin, qemu-devel

With icount shift=auto, the QEMU_CLOCK_VIRTUAL can be adjusted backwards
after a warp, resulting in the machine observing time going backwards.
Linux on powerpc can't handle this, and it results in lockup due to a
time delta underflowing.

This patch seems to solve it, no more backwards time observed. Is it a
reasonable way to address it?

Thanks,
Nick

---
 softmmu/icount.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/softmmu/icount.c b/softmmu/icount.c
index 7ae6544909..a6d605dfad 100644
--- a/softmmu/icount.c
+++ b/softmmu/icount.c
@@ -273,6 +273,9 @@ static void icount_warp_rt(void)
              */
             int64_t cur_icount = icount_get_locked();
             int64_t delta = clock - cur_icount;
+            if (delta < 0) {
+                warp_delta = 0;
+            }
             warp_delta = MIN(warp_delta, delta);
         }
         qatomic_set_i64(&timers_state.qemu_icount_bias,
-- 
2.40.1



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