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[Qemu-devel] [PATCH 0/5] mc146818rtc: fix Windows VM clock faster

From: guangrong . xiao
Subject: [Qemu-devel] [PATCH 0/5] mc146818rtc: fix Windows VM clock faster
Date: Wed, 12 Apr 2017 17:51:06 +0800

From: Xiao Guangrong <address@hidden>

We noticed that the clock on some windows VMs, e.g, Window7 and window8
is really faster and the issue can be easily reproduced by staring the
VM with '-rtc base=localtime,clock=vm,driftfix=slew -no-hpet' and 
running attached code in the guest

The root cause is that the clock will be lost if the periodic period is
changed as currently code counts the next periodic time like this:
      next_irq_clock = (cur_clock & ~(period - 1)) + period;

consider the case if cur_clock = 0x11FF and period = 0x100, then the
next_irq_clock is 0x1200, however, there is only 1 clock left to trigger
the next irq. Unfortunately, Windows guests (at least Windows7) change
the period very frequently if it runs the attached code, so that the
lost clock is accumulated, the wall-time become faster and faster

The main idea to fix the issue is we use a accurate clock period to
calculate the next irq:
    next_irq_clock = cur_clock + period;

After that, it is really convenient to compensate clock if it is needed 

The code running in windows VM is attached:
// TimeInternalTest.cpp : Defines the entry point for the console application.

#include "stdafx.h"
#pragma comment(lib, "winmm")
#include <stdio.h>
#include <windows.h>

#define SWITCH_PEROID  13

int _tmain(int argc, _TCHAR* argv[])
        if (argc != 2)
                printf("parameter error!\n");
                printf("USAGE: *.exe time(ms)\n");
                printf("example: *.exe 40\n");
                return 0;
                DWORD internal = atoi((char *)argv[1]);
                DWORD count = 0;

                while (1)
                        DWORD start = timeGetTime();
                        if ((count % SWITCH_PEROID) == 0) {
        return 0;

Tai Yunfang (1):
  mc146818rtc: properly count the time for the next interrupt

Xiao Guangrong (4):
  mc146818rtc: update periodic timer only if it is needed
  mc146818rtc: fix clock lost after scaling coalesced irq
  mc146818rtc: move x86 specific code out of periodic_timer_update
  mc146818rtc: embrace all x86 specific code

 hw/timer/mc146818rtc.c | 370 ++++++++++++++++++++++++++++++++++++-------------
 1 file changed, 272 insertions(+), 98 deletions(-)


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