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From: Vitaly Kuznetsov <vkuznets@redhat.com>
To: Paolo Bonzini <pbonzini@redhat.com>, qemu-devel@nongnu.org
Cc: "Michael S. Tsirkin" <mst@redhat.com>,
	Richard Henderson <richard.henderson@linaro.org>,
	Eduardo Habkost <ehabkost@redhat.com>,
	"Dr . David Alan Gilbert" <dgilbert@redhat.com>
Subject: Re: [PATCH 0/2] i386: Fix interrupt based Async PF enablement
Date: Tue, 06 Apr 2021 13:42:35 +0200	[thread overview]
Message-ID: <87sg43d4gk.fsf@vitty.brq.redhat.com> (raw)
In-Reply-To: <ea3fc380-332f-b9e5-aa31-3540916cfc33@redhat.com>

Paolo Bonzini <pbonzini@redhat.com> writes:

> On 01/04/21 17:19, Vitaly Kuznetsov wrote:
>> I noticed two issues with 'kvm-asyncpf-int' enablement:
>> 1) We forgot to add to to kvm_default_props[] so it doesn't get enabled
>>   automatically (unless '-cpu host' is used or the feature is enabled
>>   manually on the command line)
>> 2) We forgot to disable it for older machine types to preserve migration.
>>   This went unnoticed because of 1) I believe.
>> Vitaly Kuznetsov (2):
>>    i386: Add 'kvm-asyncpf-int' to kvm_default_props array
>>    i386: Disable 'kvm-asyncpf-int' feature for machine types <= 5.1
>>   hw/i386/pc.c      | 1 +
>>   target/i386/cpu.c | 1 +
>>   2 files changed, 2 insertions(+)
> Wasn't this intentional to avoid requiring a new kernel version?

I think I forgot the initial plan :-( The problem is that after we
disabled the original APF (#PF based) almost nobody is using the feature
as it needs to be enabled explicitly on the command line.

Several considerations regarding the default: if your kernel doesn't
support the feature you get as much as a warning:

qemu-system-x86_64: warning: host doesn't support requested feature:
CPUID.40000001H:EAX.kvm-asyncpf-int [bit 14]

older machine types are still available (I disable it for <= 5.1 but we
can consider disabling it for 5.2 too). The feature is upstream since
Linux 5.8, I know that QEMU supports much older kernels but this doesn't
probably mean that we can't enable new KVM PV features unless all
supported kernels have it, we'd have to wait many years otherwise. 


  reply	other threads:[~2021-04-06 11:44 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-04-01 15:19 [PATCH 0/2] i386: Fix interrupt based Async PF enablement Vitaly Kuznetsov
2021-04-01 15:19 ` [PATCH 1/2] i386: Add 'kvm-asyncpf-int' to kvm_default_props array Vitaly Kuznetsov
2021-04-01 15:19 ` [PATCH 2/2] i386: Disable 'kvm-asyncpf-int' feature for machine types <= 5.1 Vitaly Kuznetsov
2021-04-01 15:57 ` [PATCH 0/2] i386: Fix interrupt based Async PF enablement Paolo Bonzini
2021-04-06 11:42   ` Vitaly Kuznetsov [this message]
2021-04-08 12:46     ` Paolo Bonzini
2021-04-15 19:14       ` Dr. David Alan Gilbert
2021-04-20 17:35         ` Eduardo Habkost
2021-04-21  8:38           ` Vitaly Kuznetsov
2021-04-21  8:50             ` Daniel P. Berrangé
2021-04-21  9:23               ` Dr. David Alan Gilbert
2021-04-21  9:29               ` Vitaly Kuznetsov
2021-04-21  9:34                 ` Dr. David Alan Gilbert
2021-04-21  9:37                 ` Daniel P. Berrangé
2021-04-21  9:48                   ` Vitaly Kuznetsov

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