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Message-ID: <CAAhV-H7h1u4DiYMyWqitxhsBogfen81hOBM5UbrqHFn3+rhO2A@mail.gmail.com>
Date: Thu, 27 Nov 2025 22:38:01 +0800
From: Huacai Chen <chenhuacai@...nel.org>
To: Bibo Mao <maobibo@...ngson.cn>
Cc: "open list:LOONGARCH" <loongarch@...ts.linux.dev>, Paolo Bonzini <pbonzini@...hat.com>,
Sean Christopherson <seanjc@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/6] KVM: LoongArch: selftests: Add timer test case
On Thu, Nov 27, 2025 at 9:02 PM Bibo Mao <maobibo@...ngson.cn> wrote:
>
>
>
> On 2025/11/27 下午3:11, Huacai Chen wrote:
> > On Thu, Nov 27, 2025 at 3:03 PM Bibo Mao <maobibo@...ngson.cn> wrote:
> >>
> >>
> >>
> >> On 2025/11/27 下午2:54, Huacai Chen wrote:
> >>> On Thu, Nov 27, 2025 at 2:48 PM Bibo Mao <maobibo@...ngson.cn> wrote:
> >>>>
> >>>>
> >>>>
> >>>> On 2025/11/27 下午2:42, Huacai Chen wrote:
> >>>>> On Thu, Nov 27, 2025 at 2:21 PM Bibo Mao <maobibo@...ngson.cn> wrote:
> >>>>>>
> >>>>>>
> >>>>>>
> >>>>>> On 2025/11/27 上午10:51, Huacai Chen wrote:
> >>>>>>> On Thu, Nov 27, 2025 at 10:48 AM Bibo Mao <maobibo@...ngson.cn> wrote:
> >>>>>>>>
> >>>>>>>>
> >>>>>>>>
> >>>>>>>> On 2025/11/27 上午10:45, Huacai Chen wrote:
> >>>>>>>>> On Thu, Nov 27, 2025 at 10:37 AM Bibo Mao <maobibo@...ngson.cn> wrote:
> >>>>>>>>>>
> >>>>>>>>>>
> >>>>>>>>>>
> >>>>>>>>>> On 2025/11/27 上午10:09, Huacai Chen wrote:
> >>>>>>>>>>> On Thu, Nov 27, 2025 at 9:08 AM Bibo Mao <maobibo@...ngson.cn> wrote:
> >>>>>>>>>>>>
> >>>>>>>>>>>>
> >>>>>>>>>>>>
> >>>>>>>>>>>> On 2025/11/26 下午9:43, Huacai Chen wrote:
> >>>>>>>>>>>>> On Mon, Nov 24, 2025 at 10:17 AM Bibo Mao <maobibo@...ngson.cn> wrote:
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>> On 2025/11/24 上午10:03, Huacai Chen wrote:
> >>>>>>>>>>>>>>> On Mon, Nov 24, 2025 at 9:58 AM Bibo Mao <maobibo@...ngson.cn> wrote:
> >>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>> On 2025/11/21 下午10:08, Huacai Chen wrote:
> >>>>>>>>>>>>>>>>> Hi, Bibo,
> >>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>> On Thu, Nov 20, 2025 at 2:58 PM Bibo Mao <maobibo@...ngson.cn> wrote:
> >>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>> This patchset adds timer test case for LoongArch system, it is based
> >>>>>>>>>>>>>>>>>> on common arch_timer test case. And it includes one-shot and period mode
> >>>>>>>>>>>>>>>>>> timer interrupt test, software emulated timer function and time counter
> >>>>>>>>>>>>>>>>>> test.
> >>>>>>>>>>>>>>>>> I test this series on top of 6.18-rc6 with Loongson-3A5000, sometimes
> >>>>>>>>>>>>>>>>> it passes, sometimes I get:
> >>>>>>>>>>>>>>>>> [root@...ora kvm]# ./arch_timer
> >>>>>>>>>>>>>>>>> Random seed: 0x6b8b4567
> >>>>>>>>>>>>>>>>> Guest assert failed, vcpu 2; stage; 0; iter: 1
> >>>>>>>>>>>>>>>>> ==== Test Assertion Failure ====
> >>>>>>>>>>>>>>>>> loongarch/arch_timer.c:79: irq_iter == 0
> >>>>>>>>>>>>>>>>> pid=60138 tid=60142 errno=4 - Interrupted system call
> >>>>>>>>>>>>>>>>> 1 0x00000001200037cf: test_vcpu_run 于 arch_timer.c:70
> >>>>>>>>>>>>>>>>> 2 0x00007ffff2449f27: ?? ??:0
> >>>>>>>>>>>>>>>>> 3 0x00007ffff24c0633: ?? ??:0
> >>>>>>>>>>>>>>>>> irq_iter = 0x1.
> >>>>>>>>>>>>>>>>> Guest period timer interrupt was not triggered within the specified
> >>>>>>>>>>>>>>>>> interval, try to increase the error margin by [-e] option.
> >>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>> Is this as expected, or something is wrong?
> >>>>>>>>>>>>>>>> There is problem with that. In generic the vCPU task is rescheduled on
> >>>>>>>>>>>>>>>> other CPUs or preempted, so period timer interrupt is not handled in
> >>>>>>>>>>>>>>>> specified time.
> >>>>>>>>>>>>>>> Then this series need to be updated, or problem comes from other places?
> >>>>>>>>>>>>>> I think this series need be updated, test success criteria with period
> >>>>>>>>>>>>>> timer need consider this situation. Let me check how to handle this.
> >>>>>>>>>>>>> Any updates available?
> >>>>>>>>>>>> It can be solved by modifying udelay() method with get_cycles() or using
> >>>>>>>>>>>> cpu loop calculation method.
> >>>>>>>>>>>>
> >>>>>>>>>>>> --- a/tools/testing/selftests/kvm/include/loongarch/arch_timer.h
> >>>>>>>>>>>> +++ b/tools/testing/selftests/kvm/include/loongarch/arch_timer.hYes, no common part for it, but it can be a common problem. If other
> >>>>>>>> architectures have problems they should also modify their own
> >>>>>>>> __delay(), right?
> >>>>>>>>>>>> @@ -71,10 +71,17 @@ static inline void timer_irq_disable(void)
> >>>>>>>>>>>>
> >>>>>>>>>>>> static inline void __delay(uint64_t cycles)
> >>>>>>>>>>>> {
> >>>>>>>>>>>> - uint64_t start = timer_get_cycles();
> >>>>>>>>>>>> -
> >>>>>>>>>>>> - while ((timer_get_cycles() - start) < cycles)
> >>>>>>>>>>>> - cpu_relax();
> >>>>>>>>>>>> + uint64_t start, next, loops = 0;
> >>>>>>>>>>>> +
> >>>>>>>>>>>> + start = timer_get_cycles();
> >>>>>>>>>>>> + while (loops < cycles) {
> >>>>>>>>>>>> + next = timer_get_cycles();
> >>>>>>>>>>>> + /* only count one cycle if VM is preempted */
> >>>>>>>>>>>> + if (next > start) {
> >>>>>>>>>>>> + loops++;
> >>>>>>>>>>>> + start = next;
> >>>>>>>>>>>> + }
> >>>>>>>>>>>> + }
> >>>>>>>>>>>> }
> >>>>>>>>>>> Looks good. But ARM64 and RISC-V also use a simple implementation of
> >>>>>>>>>> there is no period test on them.
> >>>>>>>>> I think the one-shot test can also have this problem if the CPU is
> >>>>>>>>> preempted for a very long time.
> >>>>>>>>>
> >>>>>>>>>>> __delay(). So should this problem be thought of as a common problem?
> >>>>>>>>>>> If yes, maybe we can keep __delay() as is and wait for the common
> >>>>>>>>>>> parts to be fixed.
> >>>>>>>>>> Also there is no common udelay() API, it is arch specific. Someone may
> >>>>>>>>>> argue that skipping stolen cycles is not generic for __delay(), other
> >>>>>>>>>> test cases want accurate cycles rather than skipping stolen cycles. It
> >>>>>>>>>> is timer test case specific.
> >>>>>>>>>>
> >>>>>>>>>> Or adding another api __delay_loops() or keep it as is and wait for
> >>>>>>>>>> other architectures, there should be no common part for it.
> >>>>>>>>> Yes, no common part for it, but it can be a common problem. If other
> >>>>>>>>> architectures have problems they should also modify their own
> >>>>>>>>> __delay(), right?
> >>>>>>>> yes, what to do then?
> >>>>>>> Merge window is coming, let's keep it as is. And this problem only
> >>>>>>> exist when the background load is high (so preemption happens easily),
> >>>>>>> I think this is not the usual case.
> >>>>>> Another method is to modify period timer test case, calling udelay with
> >>>>>> loop times rather than one time, something like this:
> >>>>>> --- a/tools/testing/selftests/kvm/loongarch/arch_timer.c
> >>>>>> +++ b/tools/testing/selftests/kvm/loongarch/arch_timer.c
> >>>>>> @@ -75,7 +75,7 @@ static void guest_test_oneshot_timer(uint32_t cpu)
> >>>>>>
> >>>>>> static void guest_test_period_timer(uint32_t cpu)
> >>>>>> {
> >>>>>> - uint32_t irq_iter;
> >>>>>> + uint32_t irq_iter, config_iter;
> >>>>>> uint64_t us;
> >>>>>> struct test_vcpu_shared_data *shared_data = &vcpu_shared_data[cpu];
> >>>>>>
> >>>>>> @@ -84,7 +84,8 @@ static void guest_test_period_timer(uint32_t cpu)
> >>>>>> us = msecs_to_usecs(test_args.timer_period_ms) +
> >>>>>> test_args.timer_err_margin_us;
> >>>>>> timer_set_next_cmp_ms(test_args.timer_period_ms, true);
> >>>>>> /* Setup a timeout for the interrupt to arrive */
> >>>>>> - udelay(us * test_args.nr_iter);
> >>>>>> + for (config_iter = 0; config_iter < test_args.nr_iter;
> >>>>>> config_iter++)
> >>>>>> + udelay(us);
> >>>>> This can reduce the probability but still cause problem if the
> >>>>> background CPU load is very high. So I suggest keep it as is.
> >>>> If merge window is close, one fix can be post in later.
> >>>>
> >>>> Even if the background CPU load is high, timer interrupt will happen
> >>>> every time udelay() is called. So total times of timer interrupt
> >>>> triggered will meet the test case requirements.
> >>> I said it only "reduce the probability" because the one-shot test case
> >>> is also observed errors. If we use "retry method" to fix, then
> >>> one-shot test case is also needed.
> >> one-shot test case has already used split udelay() method. I observed
> >> one-shot test case failure on 3D6000 also, it is KVM timer emulation issue.
> > Yes, one-shot test case use udelay() in a loop, but it also calls
> > __GUEST_ASSERT() in a loop. So it reports errors even if only one of
> > udelay() timeouts.
> Sorry, I do not understand what is your meaning. what is the problem
> with one-shot test case? Do you mean there is problem with one-shot
> testcase also?
Sorry for confusing description.
Yes, one-shot test has problems and the reason is similar to period
test, if I understand correctly.
1. The error report comes from __GUEST_ASSERT(), right?
2. The reason of period test error is interrupt hasn't triggered
during udelay(), right?
3. For the one-shot test, there is also a udelay() and a
__GUEST_ASSERT() in every iteration, right?
4. In any iteration of the loop, if a interrupt hasn't triggered
during udelay(), __GUEST_ASSERT() will report errors, right?
Huacai
>
> Regards
> Bibo Mao
> >
> > Huacai
> >
> >>
> >> Well, let's keep it as is for the present.
> >>>
> >>> Huacai
> >>>
> >>>>
> >>>> Regards
> >>>> Bibo Mao
> >>>>>
> >>>>>
> >>>>> Huacai
> >>>>>
> >>>>>>
> >>>>>>
> >>>>>> Regards
> >>>>>> Bibo Mao
> >>>>>>>
> >>>>>>> Huacai
> >>>>>>>
> >>>>>>>>
> >>>>>>>>>
> >>>>>>>>> Huacai
> >>>>>>>>>
> >>>>>>>>>>
> >>>>>>>>>> Regards
> >>>>>>>>>> Bibo Mao
> >>>>>>>>>>>
> >>>>>>>>>>> Huacai
> >>>>>>>>>>>
> >>>>>>>>>>>>
> >>>>>>>>>>>> Regards
> >>>>>>>>>>>> Bibo Mao
> >>>>>>>>>>>>>
> >>>>>>>>>>>>> Huacai
> >>>>>>>>>>>>>
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>>> Regards
> >>>>>>>>>>>>>> Bibo Mao
> >>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>> Huacai
> >>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>> Regards
> >>>>>>>>>>>>>>>> Bibo Mao
> >>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>> Hucai
> >>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>> ---
> >>>>>>>>>>>>>>>>>> v2 ... v3:
> >>>>>>>>>>>>>>>>>> 1. Adjust order about patch 2 and patch 3
> >>>>>>>>>>>>>>>>>> 2. Add test case with alphabetical order
> >>>>>>>>>>>>>>>>>> 3. Merge one-shot and period timer interrupt test case into one
> >>>>>>>>>>>>>>>>>> 4. Only add LoongArch specific modification with common file
> >>>>>>>>>>>>>>>>>> Makefile.kvm
> >>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>> v1 ... v2:
> >>>>>>>>>>>>>>>>>> 1. Restore PC and PRMD after exception handler
> >>>>>>>>>>>>>>>>>> 2. Split patch 4 into two small patches with period timer test and
> >>>>>>>>>>>>>>>>>> time counter test
> >>>>>>>>>>>>>>>>>> 3. With time counter test, set time count with 0 when create VM. And
> >>>>>>>>>>>>>>>>>> verify time count starts from 0 in guest code
> >>>>>>>>>>>>>>>>>> ---
> >>>>>>>>>>>>>>>>>> Bibo Mao (6):
> >>>>>>>>>>>>>>>>>> KVM: LoongArch: selftests: Add system registers save and restore on
> >>>>>>>>>>>>>>>>>> exception
> >>>>>>>>>>>>>>>>>> KVM: LoongArch: selftests: Add basic interfaces
> >>>>>>>>>>>>>>>>>> KVM: LoongArch: selftests: Add exception handler register interface
> >>>>>>>>>>>>>>>>>> KVM: LoongArch: selftests: Add timer interrupt test case
> >>>>>>>>>>>>>>>>>> KVM: LoongArch: selftests: Add SW emulated timer test
> >>>>>>>>>>>>>>>>>> KVM: LoongArch: selftests: Add time counter test
> >>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>> tools/testing/selftests/kvm/Makefile.kvm | 1 +
> >>>>>>>>>>>>>>>>>> .../kvm/include/loongarch/arch_timer.h | 84 ++++++++
> >>>>>>>>>>>>>>>>>> .../kvm/include/loongarch/processor.h | 81 +++++++-
> >>>>>>>>>>>>>>>>>> .../selftests/kvm/lib/loongarch/exception.S | 6 +
> >>>>>>>>>>>>>>>>>> .../selftests/kvm/lib/loongarch/processor.c | 47 ++++-
> >>>>>>>>>>>>>>>>>> .../selftests/kvm/loongarch/arch_timer.c | 194 ++++++++++++++++++
> >>>>>>>>>>>>>>>>>> 6 files changed, 410 insertions(+), 3 deletions(-)
> >>>>>>>>>>>>>>>>>> create mode 100644 tools/testing/selftests/kvm/include/loongarch/arch_timer.h
> >>>>>>>>>>>>>>>>>> create mode 100644 tools/testing/selftests/kvm/loongarch/arch_timer.c
> >>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>>> base-commit: 23cb64fb76257309e396ea4cec8396d4a1dbae68
> >>>>>>>>>>>>>>>>>> --
> >>>>>>>>>>>>>>>>>> 2.39.3
> >>>>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>>>
> >>>>>>>>>>>>>>
> >>>>>>>>>>>>
> >>>>>>>>>>>>
> >>>>>>>>>>
> >>>>>>>>
> >>>>>>>>
> >>>>>>
> >>>>>>
> >>>>
> >>
> >>
>
>
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