Hi Bijan,
On Tue, Nov 06, 2018 at 04:32:27PM -0800, Bijan Mottahedeh wrote:
> This patch series address two Qemu issues:
This series should primarily go to qemu-devel (as it is a QEMU patch).
Could you please re-send the series to qemu-devel. Keeping the kvmarm
list on cc is nice, but only a limited set of people following KVM/Arm
development is actively reviewing QEMU patches.
Thanks,
Christoffer
>
> - improper system clock frequency initialization
> - lack of pause (virtsh suspend) time accounting
>
> A simple test to reproduce the problem executes one or more instances
> of the following command in the guest:
>
> dd if=/dev/zero of=/dev/null &
>
> and then pauses and resumes the guest after a certain delay:
>
> virsh suspend <guest> # pauses the guest
> sleep 120
> virsh resume <guest>
>
> After the guest is resumed, there are soft lockup warning messages
> displayed on the console.
>
> A comparison with x86 shows that hwclock and date values diverge after
> the above pause and resume sequence for x86 but remain the same for Arm.
>
> Patch 1 intializes the system clock frequency in Qemu similar to the
> kernel.
>
> Patch 2 accumulates the total guest pause time in QEMU and adjusts the
> virtual offset counter accordingly before the guest is resumed.
>
> The patches have been tested on an Ampere system. With the patches the
> time behavior is the same as x86 and the soft lockup messages go away.
>
>
> Clock Frequency Initialization
> ==============================
>
> Arm v8 provides the virtual counter (cntvct), virtual counter offset
> (cntvoff), and counter frequency (cntfrq) registers for guest time
> management.
>
> Linux Arm platform code initializes the system clock frequency from
> cntrfq_el0 register and sets the value into a statically created device
> tree (DT) node. It is not clear why the timer device node is created
> with TIMER_OF_DECLARE(). The DT passed from Qemu to the kernel does not
> contain a timer node.
>
> drivers/clocksource/arm_arch_timer.c:
>
> static inline u32 arch_timer_get_cntfrq(void)
> {
> return read_sysreg(cntfrq_el0);
> }
>
> rate = arch_timer_get_cntfrq();
> arch_timer_of_configure_rate(rate, np);
>
> /*
> * For historical reasons, when probing with DT we use whichever (non-zero)
> * rate was probed first, and don't verify that others match. If the first
> node
> * probed has a clock-frequency property, this overrides the HW register.
> */
> static void arch_timer_of_configure_rate(u32 rate, struct device_node *np)
> {
> ...
> if (of_property_read_u32(np, "clock-frequency", &arch_timer_rate)) {
> arch_timer_rate = rate;
> ...
> }
>
> TIMER_OF_DECLARE(armv7_arch_timer, "arm,armv7-timer", arch_timer_of_init);
> TIMER_OF_DECLARE(armv8_arch_timer, "arm,armv8-timer", arch_timer_of_init);
>
>
> Linux then initializes the clock frequency to 50MHZ.
>
> Qemu however hard codes the clock frequency to 62.5MHZ.
>
> target/arm/cpu.h:
>
> /* Scale factor for generic timers, ie number of ns per tick.
> * This gives a 62.5MHz timer.
> */
> #define GTIMER_SCALE 16
>
> The suggested fix is to follow the kernel's arch_timer_get_cntfrq()
> approach in order to set system_clock_scale to match the kernel's idea
> of clock-frequency, rather than using a hard-coded value.
>
> Ultimately, it seems that Qemu should construct the timer DT node and
> pass the actual clock frequency value to the kernel that way but that
> brings up an interface and backward compatibility considerations.
> Furthermore, the implications for ACPI method of probing is not clear.
>
>
> Pause Time Accounting
> =====================
>
> Linux registers two clock sources, a platform-independent jiffies
> clocksource and a Arm-specific arch_sys_counter; the read interface
> for the latter reads the virtual counter register:
>
> static struct clocksource clocksource_jiffies = {
> .name = "jiffies",
> .rating = 1, /* lowest valid rating*/
> .read = jiffies_read,
> .mask = CLOCKSOURCE_MASK(32),
> .mult = TICK_NSEC << JIFFIES_SHIFT, /* details above */
> .shift = JIFFIES_SHIFT,
> .max_cycles = 10,
> };
>
> static struct clocksource clocksource_counter = {
> .name = "arch_sys_counter",
> .rating = 400,
> .read = arch_counter_read,
> .mask = CLOCKSOURCE_MASK(56),
> .flags = CLOCK_SOURCE_IS_CONTINUOUS,
> };
>
> arch_counter_read()
> -> arch_timer_read_counter()
> -> arch_counter_get_cntvct()
> -> arch_timer_reg_read_stable(cntvct_el0)
>
> The virtual counter offset register is set from:
>
> kvm_timer_vcpu_load()
> -> set_cntvoff()
>
> The counter is zeroed from:
>
> kvm_timer_vcpu_put()
> -> set_cntvoff()
>
> /*
> * The kernel may decide to run userspace after calling vcpu_put, so
> * we reset cntvoff to 0 to ensure a consistent read between user
> * accesses to the virtual counter and kernel access to the physical
> * counter of non-VHE case. For VHE, the virtual counter uses a fixed
> * virtual offset of zero, so no need to zero CNTVOFF_EL2 register.
> */
> if (!has_vhe())
> set_cntvoff(0);
>
> The virtual counter offset is not modified anywhere however to account
> for pause time. The suggested fix is to add pause time accounting to
> Qemu.
>
> One potential issue is whether modifying the virtual counter offset
> breaks any assumptions, e.g., see the kvm_timer_vcpu_put() comment above.
>
>
> hwclock vs. date
> ================
>
> The hwclock on the ends up in drivers/rtc/rtc-pl031.c
>
> Real Time Clock interface for ARM AMBA PrimeCell 031 RTC
>
> ioctl("/dev/rtc", RTC_RD_TIME, ...)
> -> rtc_dev_ioctl()
> -> rtc_read_time()
> -> __rtc_read_time()
> -> pl031_read_time()
>
>
> The date command reads time from from a vdso page updated as follows:
>
> irq_enter()
> -> tick_irq_enter()
> -> tick_nohz_irq_enter()
> -> tick_nohz_update_jiffies()
> -> tick_do_update_jiffies64()
> -> tick_do_update_jiffies64()
> -> update_wall_time()
> -> timekeeping_advance()
> -> timekeeping_update()
> -> update_vsyscall(struct timekeepr *tk)
>
> The struct timekeeper uses the Arm platform clocksource_counter noted above:
>
> (gdb) p tk->tkr_mono
> $4 = {clock = 0xffff0000097ddad0 <clocksource_counter>,
> mask = 72057594037927935, cycle_last = 271809294296, mult = 335544320,
> shift = 24, xtime_nsec = 3456106496000000, base = 5435795172160,
> base_real = 1539126455000000000}
>
> This would explain why without any pause time accounting, the both
> hwclock and date command show the same time after the guest is resume
> from a pause, e.g. with the following sequence:
>
> virsh suspend <guest>
> sleep <seconds>
> virsh resume <guest>
>
> Bijan Mottahedeh (2):
> arm/virt: Initialize generic timer scale factor dynamically
> arm/virt: Account for guest pause time
>
> hw/arm/virt.c | 15 +++++++++++++++
> hw/intc/arm_gicv3_kvm.c | 39 +++++++++++++++++++++++++++++++++++++++
> target/arm/cpu.h | 3 +++
> target/arm/helper.c | 19 ++++++++++++++++---
> target/arm/internals.h | 8 ++++++--
> target/arm/kvm64.c | 1 +
> 6 files changed, 80 insertions(+), 5 deletions(-)
>
> --
> 1.8.3.1
>
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