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Message-Id: <CZYN2DBLH8Q2.U5H1A6VDAUY7@wheely>
Date: Thu, 21 Mar 2024 00:15:01 +1000
From: "Nicholas Piggin" <npiggin@...il.com>
To: "Gautam Menghani" <gautam@...ux.ibm.com>, <mpe@...erman.id.au>,
<christophe.leroy@...roup.eu>, <aneesh.kumar@...nel.org>,
<naveen.n.rao@...ux.ibm.com>
Cc: <linuxppc-dev@...ts.ozlabs.org>, <kvm@...r.kernel.org>,
<linux-kernel@...r.kernel.org>, "Vaibhav Jain" <vaibhav@...ux.ibm.com>
Subject: Re: [PATCH] arch/powerpc/kvm: Add support for reading VPA counters
for pseries guests
On Wed Mar 20, 2024 at 12:28 AM AEST, Gautam Menghani wrote:
> PAPR hypervisor has introduced three new counters in the VPA area of
> LPAR CPUs for KVM L2 guest (see [1] for terminology) observability - 2
> for context switches from host to guest and vice versa, and 1 counter
> for getting the total time spent inside the KVM guest. Add a tracepoint
> that enables reading the counters for use by ftrace/perf. Note that this
> tracepoint is only available for nestedv2 API (i.e, KVM on PowerVM).
>
> [1] Terminology:
> a. L1 refers to the VM (LPAR) booted on top of PAPR hypervisor
> b. L2 refers to the KVM guest booted on top of L1.
>
> Signed-off-by: Vaibhav Jain <vaibhav@...ux.ibm.com>
> Signed-off-by: Gautam Menghani <gautam@...ux.ibm.com>
> ---
> arch/powerpc/include/asm/kvm_host.h | 5 +++++
> arch/powerpc/include/asm/lppaca.h | 11 ++++++++---
> arch/powerpc/kvm/book3s_hv.c | 20 ++++++++++++++++++++
> arch/powerpc/kvm/trace_hv.h | 24 ++++++++++++++++++++++++
> 4 files changed, 57 insertions(+), 3 deletions(-)
>
> diff --git a/arch/powerpc/include/asm/kvm_host.h b/arch/powerpc/include/asm/kvm_host.h
> index 8abac5321..26d7bb4b9 100644
> --- a/arch/powerpc/include/asm/kvm_host.h
> +++ b/arch/powerpc/include/asm/kvm_host.h
> @@ -847,6 +847,11 @@ struct kvm_vcpu_arch {
> gpa_t nested_io_gpr;
> /* For nested APIv2 guests*/
> struct kvmhv_nestedv2_io nestedv2_io;
> +
> + /* For VPA counters having context switch and guest run time info (in ns) */
> + u64 l1_to_l2_cs;
> + u64 l2_to_l1_cs;
> + u64 l2_runtime;
> #endif
>
> #ifdef CONFIG_KVM_BOOK3S_HV_EXIT_TIMING
These aren't required here if it's just used for tracing over
a single run vcpu call are they?
> diff --git a/arch/powerpc/include/asm/lppaca.h b/arch/powerpc/include/asm/lppaca.h
> index 61ec2447d..bda6b86b9 100644
> --- a/arch/powerpc/include/asm/lppaca.h
> +++ b/arch/powerpc/include/asm/lppaca.h
> @@ -62,7 +62,8 @@ struct lppaca {
> u8 donate_dedicated_cpu; /* Donate dedicated CPU cycles */
> u8 fpregs_in_use;
> u8 pmcregs_in_use;
> - u8 reserved8[28];
> + u8 l2_accumul_cntrs_enable; /* Enable usage of counters for KVM guest */
> + u8 reserved8[27];
> __be64 wait_state_cycles; /* Wait cycles for this proc */
> u8 reserved9[28];
> __be16 slb_count; /* # of SLBs to maintain */
> @@ -92,9 +93,13 @@ struct lppaca {
> /* cacheline 4-5 */
>
> __be32 page_ins; /* CMO Hint - # page ins by OS */
> - u8 reserved12[148];
> + u8 reserved12[28];
> + volatile __be64 l1_to_l2_cs_tb;
> + volatile __be64 l2_to_l1_cs_tb;
> + volatile __be64 l2_runtime_tb;
> + u8 reserved13[96];
> volatile __be64 dtl_idx; /* Dispatch Trace Log head index */
> - u8 reserved13[96];
> + u8 reserved14[96];
> } ____cacheline_aligned;
>
> #define lppaca_of(cpu) (*paca_ptrs[cpu]->lppaca_ptr)
> diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
> index 2b04eba90..b94461b5f 100644
> --- a/arch/powerpc/kvm/book3s_hv.c
> +++ b/arch/powerpc/kvm/book3s_hv.c
> @@ -4092,6 +4092,7 @@ static int kvmhv_vcpu_entry_nestedv2(struct kvm_vcpu *vcpu, u64 time_limit,
> unsigned long msr, i;
> int trap;
> long rc;
> + struct lppaca *lp = get_lppaca();
Does get_lppaca() emit some inline asm that can't be optimised?
Could move it under the unlikely branches if so.
>
> io = &vcpu->arch.nestedv2_io;
>
KVM L0 could in theory provide this for v1 L1s too, so could this
be done at a higher level to cover both?
> @@ -4107,6 +4108,17 @@ static int kvmhv_vcpu_entry_nestedv2(struct kvm_vcpu *vcpu, u64 time_limit,
> kvmppc_gse_put_u64(io->vcpu_run_input, KVMPPC_GSID_LPCR, lpcr);
>
> accumulate_time(vcpu, &vcpu->arch.in_guest);
> +
> + /* Reset the guest host context switch timing */
> + if (unlikely(trace_kvmppc_vcpu_exit_cs_time_enabled())) {
> + lp->l2_accumul_cntrs_enable = 1;
> + lp->l1_to_l2_cs_tb = 0;
> + lp->l2_to_l1_cs_tb = 0;
> + lp->l2_runtime_tb = 0;
> + } else {
> + lp->l2_accumul_cntrs_enable = 0;
> + }
Instead of zeroing here zero after the exit, which avoids the
else branch and possibly avoids an obscure race with the counters.
What if trace_kvmppc_vcpu_exit_cs_time_enabled() is false here...
> +
> rc = plpar_guest_run_vcpu(0, vcpu->kvm->arch.lpid, vcpu->vcpu_id,
> &trap, &i);
>
> @@ -4133,6 +4145,14 @@ static int kvmhv_vcpu_entry_nestedv2(struct kvm_vcpu *vcpu, u64 time_limit,
>
> timer_rearm_host_dec(*tb);
>
> + /* Record context switch and guest_run_time data */
> + if (unlikely(trace_kvmppc_vcpu_exit_cs_time_enabled())) {
> + vcpu->arch.l1_to_l2_cs = tb_to_ns(be64_to_cpu(lp->l1_to_l2_cs_tb));
> + vcpu->arch.l2_to_l1_cs = tb_to_ns(be64_to_cpu(lp->l2_to_l1_cs_tb));
> + vcpu->arch.l2_runtime = tb_to_ns(be64_to_cpu(lp->l2_runtime_tb));
> + trace_kvmppc_vcpu_exit_cs_time(vcpu);
> + }
.. and true here. If it had been previously true then it would trace
stale values I think?
Would something like this work?
if (unlikely(trace_kvmppc_vcpu_exit_cs_time_enabled()))
get_lppaca()->l2_accumul_cntrs_enable = 1;
[run vcpu ; ...]
if (unlikely(trace_kvmppc_vcpu_exit_cs_time_enabled()))
do_trace_nested_cs_time(vcpu);
...
static void do_trace_nested_cs_time(struct vcpu *vcpu)
{
struct lppaca *lp = get_lppaca();
u64 l1_to_l2, l2_to_l1, l2_runtime;
if (!lp->l2_accumul_cntrs_enable)
return;
l1_to_l2 = tb_to_ns(be64_to_cpu(lp->l1_to_l2_cs_tb));
l2_to_l1 = tb_to_ns(be64_to_cpu(lp->l2_to_l1_cs_tb));
l2_runtime = tb_to_ns(be64_to_cpu(lp->l2_runtime_tb));
trace_kvmppc_vcpu_exit_cs_time(vcpu->cpu_id, l1_to_l2,
l2_to_l1, l2_runtime);
lp->l1_to_l2_cs_tb = 0;
lp->l2_to_l1_cs_tb = 0;
lp->l2_runtime_tb = 0;
lp->l2_accumul_cntrs_enable = 0;
}
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