From: Paolo Bonzini <pbonzini@redhat.com>
To: Vitaly Kuznetsov <vkuznets@redhat.com>
Cc: linux-kernel@vger.kernel.org, Andy Lutomirski <luto@kernel.org>,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
"H. Peter Anvin" <hpa@zytor.com>,
Sean Christopherson <sean.j.christopherson@intel.com>,
Wanpeng Li <wanpengli@tencent.com>,
Jim Mattson <jmattson@google.com>,
x86@kernel.org, kvm@vger.kernel.org
Subject: Re: [PATCH RFC 3/6] KVM: x86: interrupt based APF page-ready event delivery
Date: Wed, 29 Apr 2020 15:21:45 +0200 [thread overview]
Message-ID: <15c625c5-d551-85fd-8412-f88503e6a86b@redhat.com> (raw)
In-Reply-To: <87ees6h3cm.fsf@vitty.brq.redhat.com>
On 29/04/20 14:40, Vitaly Kuznetsov wrote:
> Paolo Bonzini <pbonzini@redhat.com> writes:
>
>> On 29/04/20 11:36, Vitaly Kuznetsov wrote:
>>> +
>>> + Type 1 page (page missing) events are currently always delivered as
>>> + synthetic #PF exception. Type 2 (page ready) are either delivered
>>> + by #PF exception (when bit 3 of MSR_KVM_ASYNC_PF_EN is clear) or
>>> + via an APIC interrupt (when bit 3 set). APIC interrupt delivery is
>>> + controlled by MSR_KVM_ASYNC_PF2.
>>
>> I think we should (in the non-RFC version) block async page faults
>> completely and only keep APF_HALT unless the guest is using page ready
>> interrupt delivery.
>
> Sure, we can do that. This is, however, a significant behavioral change:
> APF_HALT frees the host, not the guest, so even if the combined
> performance of all guests on the same pCPU remain the same guests with
> e.g. a lot of simultaneously running processes may suffer more.
Yes, it is a significant change. However the resulting clean up in the
spec is significant, because we don't have type 2 notifications at all
anymore.
(APF_HALT does free the guest a little bit by allowing interrupt
delivery during a host page fault; in particular it lets the scheduler
tick run, which does improve responsiveness somewhat significantly).
Likewise, I think we should clean up the guest side without prejudice.
Patch 6 should disable async page fault unless page-ready interrupts are
available, and drop the page ready case from the #PF handler.
Thanks,
Paolo
> In theory, we can keep two mechanisms side by side for as long as we
> want but if the end goal is to have '#PF abuse eliminated' than we'll
> have to get rid of the legacy one some day. The day when the new
> mechanism lands is also a good choice :-)
next prev parent reply other threads:[~2020-04-29 13:21 UTC|newest]
Thread overview: 41+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-04-29 9:36 [PATCH RFC 0/6] KVM: x86: Interrupt-based mechanism for async_pf 'page present' notifications Vitaly Kuznetsov
2020-04-29 9:36 ` [PATCH RFC 1/6] Revert "KVM: async_pf: Fix #DF due to inject "Page not Present" and "Page Ready" exceptions simultaneously" Vitaly Kuznetsov
2020-05-05 1:22 ` Gavin Shan
2020-05-05 14:16 ` Vivek Goyal
2020-05-06 15:17 ` Vitaly Kuznetsov
2020-05-11 19:17 ` Vivek Goyal
2020-04-29 9:36 ` [PATCH RFC 2/6] KVM: x86: extend struct kvm_vcpu_pv_apf_data with token info Vitaly Kuznetsov
2020-05-04 23:52 ` Gavin Shan
2020-05-05 8:08 ` Vitaly Kuznetsov
2020-04-29 9:36 ` [PATCH RFC 3/6] KVM: x86: interrupt based APF page-ready event delivery Vitaly Kuznetsov
2020-04-29 10:54 ` Paolo Bonzini
2020-04-29 12:40 ` Vitaly Kuznetsov
2020-04-29 13:21 ` Paolo Bonzini [this message]
2020-04-29 21:27 ` Peter Xu
2020-04-30 8:31 ` Vitaly Kuznetsov
2020-04-30 13:28 ` Peter Xu
2020-04-30 13:49 ` Vitaly Kuznetsov
2020-05-05 15:22 ` Vivek Goyal
2020-04-29 9:36 ` [PATCH RFC 4/6] KVM: x86: acknowledgment mechanism for async pf page ready notifications Vitaly Kuznetsov
2020-04-29 17:28 ` Andy Lutomirski
2020-04-29 17:40 ` Paolo Bonzini
2020-04-30 0:45 ` Andy Lutomirski
2020-04-30 6:46 ` Paolo Bonzini
2020-04-30 6:49 ` Paolo Bonzini
2020-04-30 8:40 ` Vitaly Kuznetsov
2020-04-30 9:27 ` Paolo Bonzini
2020-04-30 11:33 ` Vitaly Kuznetsov
2020-04-30 11:43 ` Paolo Bonzini
2020-05-05 0:36 ` Gavin Shan
2020-05-05 8:16 ` Vitaly Kuznetsov
2020-05-05 8:51 ` Gavin Shan
2020-05-05 9:54 ` Paolo Bonzini
2020-04-29 9:36 ` [PATCH RFC 5/6] KVM: x86: announce KVM_FEATURE_ASYNC_PF_INT Vitaly Kuznetsov
2020-04-29 10:40 ` Peter Zijlstra
2020-04-29 9:36 ` [PATCH RFC 6/6] KVM: x86: Switch KVM guest to using interrupts for page ready APF delivery Vitaly Kuznetsov
2020-04-29 10:53 ` Paolo Bonzini
2020-04-29 12:44 ` Vitaly Kuznetsov
2020-04-29 13:19 ` Paolo Bonzini
2020-04-29 14:34 ` Vitaly Kuznetsov
2020-05-05 18:59 ` Vivek Goyal
2020-05-05 0:42 ` Gavin Shan
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