[bug #64623] calculate tsc rate by the actual pmtimer delta

[Qi Ai]

URL:
  <https://savannah.gnu.org/bugs/?64623>

                 Summary: calculate tsc rate by the actual pmtimer delta
                   Group: GNU GRUB
               Submitter: aiqi
               Submitted: Tue 05 Sep 2023 11:18:19 AM UTC
                Category: Booting
                Severity: Major
                Priority: 5 - Normal
              Item Group: Software Error
                  Status: None
                 Privacy: Public
             Assigned to: None
         Originator Name: Qi Ai
        Originator Email: 
             Open/Closed: Open
                 Release: Git master
                 Release: 
         Discussion Lock: Any
         Reproducibility: None
         Planned Release: None


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Follow-up Comments:


-------------------------------------------------------
Date: Tue 05 Sep 2023 11:18:19 AM UTC By: Qi Ai <aiqi>
In qemu/kvm, qemu emulates the pmtimer. Reading pmtimer in vm will cause a vm
exit, and qemu read the real time. If the qemu thread is scheduled out before
reading time, the time that vm reads may delay. In some case, pmtimer delta
may be far more than 1ms, which cause tsc rate is much larger than real rate.
So in grub menu, grub timeout will take more time than expected. So calculate
tsc rate by the actual pmtimer delta, not always use 1ms.

This fixs cannot fix the issue completely, it just reduce the probability of
the issue.

diff --git a/grub-core/kern/i386/tsc_pmtimer.c
b/grub-core/kern/i386/tsc_pmtimer.c
index 5c03c510a..6d0802dfe 100644
--- a/grub-core/kern/i386/tsc_pmtimer.c
+++ b/grub-core/kern/i386/tsc_pmtimer.c
@@ -84,7 +84,7 @@ grub_pmtimer_wait_count_tsc (grub_port_t pmtimer,
                        cur - start);
          grub_dprintf ("pmtimer", "tsc delta is 0x%"PRIxGRUB_UINT64_T"\n",
                        end_tsc - start_tsc);
-         return end_tsc - start_tsc;
+         return (end_tsc - start_tsc) / (cur - start) * num_pm_ticks;
        }







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