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Re: [Qemu-devel] [PATCH RESEND v2] i386/kvm: add support for KVM_CAP_X86

From: Paolo Bonzini
Subject: Re: [Qemu-devel] [PATCH RESEND v2] i386/kvm: add support for KVM_CAP_X86_DISABLE_EXITS
Date: Wed, 16 May 2018 14:44:24 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0

On 12/05/2018 00:12, Michael S. Tsirkin wrote:
>> Maybe it's a better idea than overloading an option that is only
>> expected to control a CPUID bit.
> Well -realtime would be a bit confusing in that it enables mlock by
> default.

Currently, the only suboption of "-realtime" is mlock, which means that
the only three valid uses of it are

        -realtime mlock=on
        -realtime mlock=off
        -realtime ''

We can change the default, I think.  Only the third would change
meaning, and it's a slightly crazy way to use the option.

> From pure API point of view, hint-dedicated looks good since
> it seems to say "optimize for a dedicated host CPU" and
> it's a hint in that guests keep working if you violate this
> slightly once in a while.
> But I agree there's a problem: right now "kvm-" means "KVM PV"
> as opposed to e.g. HV enlightened gusts.
> So if you enable hyperv and also want to disable halt existing,
> what then? What should kvm-hint-dedicated=on do?

kvm-hint-dedicated=on only sets the CPUID bit, which Linux for example
uses that to disable pv spinlocks.  "-realtime dedicated-cpus=on" only
disables the vmexits.  You can use the two independently.



> So how about a new hint-dedicated=on cpu flag?  We can have that set
> kvm-hint-dedicated if kvm PV is enabled.

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