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From: Paolo Bonzini <pbonzini@redhat.com>
To: Vitaly Kuznetsov <vkuznets@redhat.com>,
	x86@kernel.org, kvm@vger.kernel.org
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>
Subject: Re: [PATCH RFC 4/6] KVM: x86: acknowledgment mechanism for async pf page ready notifications
Date: Thu, 30 Apr 2020 13:43:13 +0200	[thread overview]
Message-ID: <0ff17250-1943-837a-734d-73a8b4a97ae2@redhat.com> (raw)
In-Reply-To: <87k11xfbsh.fsf@vitty.brq.redhat.com>

On 30/04/20 13:33, Vitaly Kuznetsov wrote:
>> I would expect that it needs to keep it in a global variable anyway, but
>> yes this is a good point.  You can also keep the ACK MSR and store the
>> pending bit in the other MSR, kind of like you have separate ISR and EOI
>> registers in the LAPIC.
>>
> Honestly I was inspired by Hyper-V's HV_X64_MSR_EOM MSR as the protocol
> we're trying to come up with here is very similar to HV messaging)

Oh, that's true actually.

> I'm not exactly sure why we need the pending bit after we drop #PF. When
> we call kvm_check_async_pf_completion() from MSR_KVM_ASYNC_PF_ACK write
> it will (in case there are page ready events in the queue) check if the
> slot is empty, put one there and raise IRQ regardless of guest's current
> state. It may or may not get injected immediately but we don't care.> The second invocation of kvm_check_async_pf_completion() from vcpu_run()
> will just go away.

You're right, you can just use the value in the guest to see if the
guest is ready.  This is also similar to how #VE handles re-entrancy,
however because this is an interrupt we have IF to delay the IRQ until
after the interrupt handler has finished.

By dropping the #PF page ready case, we can also drop the ugly case
where WRMSR injects a page ready page fault even if IF=0.  That one is
safe on Linux, but Andy didn't like it.

Paolo


  reply	other threads:[~2020-04-30 11:43 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
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 [this message]
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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