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Ports/qemu: Use the coarse monotonic clock for timing CPU ticks

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While this loses quite a bit of accuracy (although to no apparent
decrease in emulation quality) , it helps avoiding the additional
overhead of the `clock_gettime` syscall (as `CLOCK_MONOTONIC_COARSE`
is forwarded using the mapped time page) and we don't have to do a
HPET timer read for each tick.

This results in a decrease of Serenity boot time from 1h16m down to
42m when running on Serenity.
This commit is contained in:
Tim Schumacher 2022-07-28 08:31:21 +02:00 committed by Brian Gianforcaro
parent a934fa3d28
commit be6b3710c8
4 changed files with 39 additions and 2 deletions
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2
Ports/qemu/patches/0001-Add-build-system-support-for-SerenityOS.patch
View file

@ -8,7 +8,7 @@ Subject: [PATCH] Add build system support for SerenityOS
1 file changed, 6 insertions(+)
diff --git a/configure b/configure
index 7c08c18..3177605 100755
index 7c08c18358becf49779c876b0f3d17329df053c6..3177605054876b387cd2b93463025ee3203991e7 100755
--- a/configure
+++ b/configure
@@ -496,6 +496,8 @@ elif check_define __NetBSD__; then

2
Ports/qemu/patches/0002-Extend-short-scan-sets-into-the-full-list.patch
View file

@ -10,7 +10,7 @@ sets, so extend them into a full list manually.
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/chardev/char.c b/chardev/char.c
index 0169d8d..03ce487 100644
index 0169d8dde4b533c9cf851831b03c8adcac24cff5..03ce487a23c92b70981643bd213930f5d074afdb 100644
--- a/chardev/char.c
+++ b/chardev/char.c
@@ -382,11 +382,11 @@ QemuOpts *qemu_chr_parse_compat(const char *label, const char *filename,

27
Ports/qemu/patches/0003-Use-the-coarse-monotonic-clock-for-timing-CPU-ticks.patch Normal file
View file

@ -0,0 +1,27 @@
From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Tim Schumacher <timschumi@gmx.de>
Date: Sat, 1 Oct 2022 02:46:48 +0200
Subject: [PATCH] Use the coarse monotonic clock for timing CPU ticks
While this loses quite a bit of accuracy (although to no apparent
decrease in emulation quality), it helps avoiding the additional
overhead of the `clock_gettime` syscall (as `CLOCK_MONOTONIC_COARSE`
is forwarded using the mapped time page) and we don't have to do a
HPET timer read for each tick.
---
include/qemu/timer.h | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)
diff --git a/include/qemu/timer.h b/include/qemu/timer.h
index ee071e07d131641131ed9705e7407f153bbf6c67..97fb9b9ac28bd36cd7200e92c3c0c1e30858aa0d 100644
--- a/include/qemu/timer.h
+++ b/include/qemu/timer.h
@@ -833,7 +833,7 @@ static inline int64_t get_clock(void)
{
if (use_rt_clock) {
struct timespec ts;
- clock_gettime(CLOCK_MONOTONIC, &ts);
+ clock_gettime(CLOCK_MONOTONIC_COARSE, &ts);
return ts.tv_sec * 1000000000LL + ts.tv_nsec;
} else {
/* XXX: using gettimeofday leads to problems if the date

10
Ports/qemu/patches/ReadMe.md
View file

@ -12,3 +12,13 @@ Extend short scan sets into the full list
We don't support the (apparently nonstandard) short variant of scan
sets, so extend them into a full list manually.
## `0003-Use-the-coarse-monotonic-clock-for-timing-CPU-ticks.patch`
Use the coarse monotonic clock for timing CPU ticks
While this loses quite a bit of accuracy (although to no apparent
decrease in emulation quality), it helps avoiding the additional
overhead of the `clock_gettime` syscall (as `CLOCK_MONOTONIC_COARSE`
is forwarded using the mapped time page) and we don't have to do a
HPET timer read for each tick.