linux-kernel - Re: [PATCH] kvm/debugfs: add file to get vcpu steal time statistics

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Message-ID: <bf5c35ab-d858-477c-9b72-95161ad72f1d@yandex-team.ru>
Date: Tue, 5 Nov 2024 15:43:58 +0300
From: Denis Plotnikov <den-plotnikov@...dex-team.ru>
To: Sean Christopherson <seanjc@...gle.com>
Cc: kvm@...r.kernel.org, pbonzini@...hat.com, yc-core@...dex-team.ru,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] kvm/debugfs: add file to get vcpu steal time statistics


 > On 9/30/24 18:44, Sean Christopherson wrote:
>>> No, I mean by using the host userspace VMA to read the memory.
>>
>> Oh, I think I got your idea. You mean
>> using KVM_CAP_X86_MSR_FILTER which...
>>
>> "In combination with KVM_CAP_X86_USER_SPACE_MSR, this allows user space to
>> trap and emulate MSRs ..."
>>
>> And then having guest's steal time struct valid address read the value from
>> userspace VMM like qemu directly.
> 
> Yep, exactly!

By the way, what if we add "steal time" as a kvm statistics item?

Why I think it's a good idea?
* it is available via standard KVM_GET_STATS_FD
* it doesn't introduce any overhead
* it is quite easy to add with just three lines of code
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -1596,6 +1596,7 @@ struct kvm_vcpu_stat {
         u64 preemption_other;
         u64 guest_mode;
         u64 notify_window_exits;
+       u64 steal_time;
  };

  struct x86_instruction_info;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 83fe0a78146fc..cd771aef1558a 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -291,6 +291,7 @@ const struct _kvm_stats_desc kvm_vcpu_stats_desc[] = {
         STATS_DESC_COUNTER(VCPU, preemption_other),
         STATS_DESC_IBOOLEAN(VCPU, guest_mode),
         STATS_DESC_COUNTER(VCPU, notify_window_exits),
+       STATS_DESC_TIME_NSEC(VCPU, steal_time),
  };

  const struct kvm_stats_header kvm_vcpu_stats_header = {
@@ -3763,6 +3764,7 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
         version += 1;
         unsafe_put_user(version, &st->version, out);

+       vcpu->stat.steal_time = steal;

The disadvantage of this approach is that it adds some kind of data 
duplication but it doesn't seem to be a problem - using shadowing and 
caching are common practices.

My concern about intercepting steal time MSR in user space is 
overcomplication - we need to add significant amount of userspace code 
to achieve what we can get in much easier and, in my opinion, cleaner 
way. I think it's a cleaner way because every userspace app (like QEMU) 
will get steal time without any modification via means provided by kvm. 
For example, QEMU will be able to get steal time via qmp with 
"query-stats" command which returns every statistics item provided by 
KVM_GET_STATS_FD.