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Re: [PATCH 0/2] i386: Fix interrupt based Async PF enablement

From: Vitaly Kuznetsov
Subject: Re: [PATCH 0/2] i386: Fix interrupt based Async PF enablement
Date: Tue, 06 Apr 2021 13:42:35 +0200

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. 


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