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Message-ID: <87o8mlooki.fsf@vitty.brq.redhat.com>
Date: Fri, 04 Sep 2020 11:53:33 +0200
From: Vitaly Kuznetsov <vkuznets@...hat.com>
To: Haiwei Li <lihaiwei.kernel@...il.com>
Cc: "pbonzini\@redhat.com" <pbonzini@...hat.com>,
Sean Christopherson <sean.j.christopherson@...el.com>,
"wanpengli\@tencent.com" <wanpengli@...cent.com>,
"jmattson\@google.com" <jmattson@...gle.com>,
"joro\@8bytes.org" <joro@...tes.org>, tglx@...utronix.de,
mingo@...hat.com, "bp\@alien8.de" <bp@...en8.de>,
"hpa\@zytor.com" <hpa@...or.com>,
"linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
"kvm\@vger.kernel.org" <kvm@...r.kernel.org>,
"x86\@kernel.org" <x86@...nel.org>
Subject: Re: [PATCH v2] KVM: Check the allocation of pv cpu mask
Haiwei Li <lihaiwei.kernel@...il.com> writes:
> On 20/9/3 18:39, Vitaly Kuznetsov wrote:
>> Haiwei Li <lihaiwei.kernel@...il.com> writes:
>>
>>> From: Haiwei Li <lihaiwei@...cent.com>
>>>
>>> check the allocation of per-cpu __pv_cpu_mask. Initialize ops only when
>>> successful.
>>>
>>> Signed-off-by: Haiwei Li <lihaiwei@...cent.com>
>>> ---
>>> arch/x86/kernel/kvm.c | 24 ++++++++++++++++++++----
>>> 1 file changed, 20 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
>>> index 08320b0b2b27..d3c062e551d7 100644
>>> --- a/arch/x86/kernel/kvm.c
>>> +++ b/arch/x86/kernel/kvm.c
>>> @@ -555,7 +555,6 @@ static void kvm_send_ipi_mask_allbutself(const
>>> struct cpumask *mask, int vector)
>>> static void kvm_setup_pv_ipi(void)
>>> {
>>> apic->send_IPI_mask = kvm_send_ipi_mask;
>>> - apic->send_IPI_mask_allbutself = kvm_send_ipi_mask_allbutself;
>>> pr_info("setup PV IPIs\n");
>>> }
>>>
>>> @@ -654,7 +653,6 @@ static void __init kvm_guest_init(void)
>>> }
>>>
>>> if (pv_tlb_flush_supported()) {
>>> - pv_ops.mmu.flush_tlb_others = kvm_flush_tlb_others;
>>> pv_ops.mmu.tlb_remove_table = tlb_remove_table;
>>> pr_info("KVM setup pv remote TLB flush\n");
>>> }
>>> @@ -767,6 +765,14 @@ static __init int activate_jump_labels(void)
>>> }
>>> arch_initcall(activate_jump_labels);
>>>
>>> +static void kvm_free_pv_cpu_mask(void)
>>> +{
>>> + unsigned int cpu;
>>> +
>>> + for_each_possible_cpu(cpu)
>>> + free_cpumask_var(per_cpu(__pv_cpu_mask, cpu));
>>> +}
>>> +
>>> static __init int kvm_alloc_cpumask(void)
>>> {
>>> int cpu;
>>> @@ -785,11 +791,21 @@ static __init int kvm_alloc_cpumask(void)
>>>
>>> if (alloc)
>>> for_each_possible_cpu(cpu) {
>>> - zalloc_cpumask_var_node(per_cpu_ptr(&__pv_cpu_mask, cpu),
>>> - GFP_KERNEL, cpu_to_node(cpu));
>>> + if (!zalloc_cpumask_var_node(
>>> + per_cpu_ptr(&__pv_cpu_mask, cpu),
>>> + GFP_KERNEL, cpu_to_node(cpu)))
>>> + goto zalloc_cpumask_fail;
>>> }
>>>
>>> +#if defined(CONFIG_SMP)
>>> + apic->send_IPI_mask_allbutself = kvm_send_ipi_mask_allbutself;
>>> +#endif
>>> + pv_ops.mmu.flush_tlb_others = kvm_flush_tlb_others;
>>
>> This is too late I'm afraid. If I'm not mistaken PV patching happens
>> earlier, so .init.guest_late_init (kvm_guest_init()) is good and
>> arch_initcall() is bad.
>
> .init.guest_late_init (kvm_guest_init()) is called before
> arch_initcall() and kvm_flush_tlb_others && kvm_send_ipi_mask_allbutself
> rely on __pv_cpu_mask. So, i can not put this assign in kvm_guest_init().
>
>>
>> Have you checked that with this patch kvm_flush_tlb_others() is still
>> being called?
>
> yes. I add a printk and i get the log.
>
This is weird. I do the following on top of your patch:
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index d3c062e551d7..f441209ff0a4 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -620,6 +620,8 @@ static void kvm_flush_tlb_others(const struct cpumask *cpumask,
struct kvm_steal_time *src;
struct cpumask *flushmask = this_cpu_cpumask_var_ptr(__pv_cpu_mask);
+ trace_printk("PV TLB flush %d CPUs\n", cpumask_weight(cpumask));
+
cpumask_copy(flushmask, cpumask);
/*
* We have to call flush only on online vCPUs. And
With your patch I don't see any calls:
# grep -c -v '^#' /sys/kernel/debug/tracing/trace
0
with your patch reverted I see them:
# grep -c -v '^#' /sys/kernel/debug/tracing/trace
4571
>>
>> Actually, there is no need to assign kvm_flush_tlb_others() so late. We
>> can always check if __pv_cpu_mask was allocated and revert back to the
>> architectural path if not.
> I am sorry i don't really understand. Can you explain in more detail? Thx.
>
I mean we can always call e.g. kvm_flush_tlb_others(), even if (very
unlikely) the mask wasn't allocated. We just need to check for
that. Something like (completely untested):
diff --git a/arch/x86/kernel/kvm.c b/arch/x86/kernel/kvm.c
index d3c062e551d7..e3676cdee6a2 100644
--- a/arch/x86/kernel/kvm.c
+++ b/arch/x86/kernel/kvm.c
@@ -620,6 +620,11 @@ static void kvm_flush_tlb_others(const struct cpumask *cpumask,
struct kvm_steal_time *src;
struct cpumask *flushmask = this_cpu_cpumask_var_ptr(__pv_cpu_mask);
+ if (unlikely(!flushmask)) {
+ flushmask = cpumask;
+ goto do_native;
+ }
+
cpumask_copy(flushmask, cpumask);
/*
* We have to call flush only on online vCPUs. And
@@ -635,6 +640,7 @@ static void kvm_flush_tlb_others(const struct cpumask *cpumask,
}
}
+do_native:
native_flush_tlb_others(flushmask, info);
}
>>
>>> return 0;
>>> +
>>> +zalloc_cpumask_fail:
>>> + kvm_free_pv_cpu_mask();
>>> + return -ENOMEM;
>>> }
>>> arch_initcall(kvm_alloc_cpumask);
>>>
>>> --
>>> 2.18.4
>>>
>>
>
--
Vitaly
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