Re: [PATCH RESEND v2 6/8] KVM: x86/pmu: Defer counter emulated overflow via pmc->stale_counter

Sean Christopherson
2022-08-30T18:00:16Z

On Tue, Aug 23, 2022, Like Xu wrote:
> From: Like Xu <likexu@tencent.com>
>
> There are contextual restrictions on the functions that can be called
> in the *_exit_handlers_fastpath path, for example calling
> pmc_reprogram_counter() brings up a host complaint like:

State the actual problem instead of forcing the reader to decipher that from the
stacktrace.

> [*] BUG: sleeping function called from invalid context at kernel/locking/mutex.c:580
> [*] in_atomic(): 1, irqs_disabled(): 1, non_block: 0, pid: 2981888, name: CPU 15/KVM
> [*] preempt_count: 1, expected: 0
> [*] RCU nest depth: 0, expected: 0
> [*] INFO: lockdep is turned off.
> [*] irq event stamp: 0
> [*] hardirqs last enabled at (0): [<0000000000000000>] 0x0
> [*] hardirqs last disabled at (0): [<ffffffff8121222a>] copy_process+0x146a/0x62d0
> [*] softirqs last enabled at (0): [<ffffffff81212269>] copy_process+0x14a9/0x62d0
> [*] softirqs last disabled at (0): [<0000000000000000>] 0x0
> [*] Preemption disabled at:
> [*] [<ffffffffc2063fc1>] vcpu_enter_guest+0x1001/0x3dc0 [kvm]
> [*] CPU: 17 PID: 2981888 Comm: CPU 15/KVM Kdump: 5.19.0-rc1-g239111db364c-dirty #2
> [*] Call Trace:
> [*] <TASK>
> [*] dump_stack_lvl+0x6c/0x9b
> [*] __might_resched.cold+0x22e/0x297
> [*] __mutex_lock+0xc0/0x23b0
> [*] perf_event_ctx_lock_nested+0x18f/0x340
> [*] perf_event_pause+0x1a/0x110
> [*] reprogram_counter+0x2af/0x1490 [kvm]
> [*] kvm_pmu_trigger_event+0x429/0x950 [kvm]
> [*] kvm_skip_emulated_instruction+0x48/0x90 [kvm]
> [*] handle_fastpath_set_msr_irqoff+0x349/0x3b0 [kvm]
> [*] vmx_vcpu_run+0x268e/0x3b80 [kvm_intel]
> [*] vcpu_enter_guest+0x1d22/0x3dc0 [kvm]
>
> A new stale_counter field is introduced to keep this part of the semantics
> invariant. It records the current counter value and it's used to determine
> whether to inject an emulated overflow interrupt in the later
> kvm_pmu_handle_event(), given that the internal count value from its
> perf_event has not been added to pmc->counter in time, or the guest
> will update the value of a running counter directly.

Describe what the change is at a high level, don't give a play-by-play of the
code changes.

Defer reprogramming counters and handling overflow via KVM_REQ_PMU
when incrementing counters. KVM skips emulated WRMSR in the VM-Exit
fastpath, the fastpath runs with IRQs disabled, skipping instructions
can increment and reprogram counters, reprogramming counters can
sleep, and sleeping is disallowed while IRQs are disabled.

<stack trace>

Add a field to kvm_pmc to track the previous counter value in order
to defer overflow detection to kvm_pmu_handle_event() (reprogramming
must be done before handling overflow).

> Opportunistically shrink sizeof(struct kvm_pmc) a bit.
>
> Suggested-by: Wanpeng Li <wanpengli@tencent.com>
> Fixes: 9cd803d496e7 ("KVM: x86: Update vPMCs when retiring instructions")
> Signed-off-by: Like Xu <likexu@tencent.com>
> ---
> arch/x86/include/asm/kvm_host.h | 5 +++--
> arch/x86/kvm/pmu.c | 15 ++++++++-------
> arch/x86/kvm/svm/pmu.c | 2 +-
> arch/x86/kvm/vmx/pmu_intel.c | 4 ++--
> 4 files changed, 14 insertions(+), 12 deletions(-)
>
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index 4e568a7ef464..ffd982bf015d 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -488,7 +488,10 @@ enum pmc_type {
> struct kvm_pmc {
> enum pmc_type type;
> u8 idx;
> + bool is_paused;
> + bool intr;
> u64 counter;
> + u64 stale_counter;

Use "prev_counter", "stale" makes it sound like a flag, e.g. "this counter is
stale".

> u64 eventsel;
> struct perf_event *perf_event;
> struct kvm_vcpu *vcpu;
> @@ -498,8 +501,6 @@ struct kvm_pmc {
> * ctrl value for fixed counters.
> */
> u64 current_config;
> - bool is_paused;
> - bool intr;
> };
>
> #define KVM_PMC_MAX_FIXED 3
> diff --git a/arch/x86/kvm/pmu.c b/arch/x86/kvm/pmu.c
> index 6940cbeee54d..45d062cb1dd5 100644
> --- a/arch/x86/kvm/pmu.c
> +++ b/arch/x86/kvm/pmu.c
> @@ -350,6 +350,12 @@ void kvm_pmu_handle_event(struct kvm_vcpu *vcpu)
> }
>
> __reprogram_counter(pmc);
> +
> + if (pmc->stale_counter) {

This check is unnecessary. The values are unsigned, so counter can't be less than
the previous value if the previous value was '0'.

> + if (pmc->counter < pmc->stale_counter)
> + __kvm_perf_overflow(pmc, false);
> + pmc->stale_counter = 0;
> + }