From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [PATCH RESEND v2] i386/kvm: add support for
 KVM_CAP_X86_DISABLE_EXITS
Date: Wed, 16 May 2018 19:21:04 +0300
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Cc: Wanpeng Li <kernellwp@gmail.com>, Paolo Bonzini <pbonzini@redhat.com>,
	qemu-devel@nongnu.org, kvm@vger.kernel.org,
	Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>
To: Eduardo Habkost <ehabkost@redhat.com>
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On Wed, May 16, 2018 at 12:33:50PM -0300, Eduardo Habkost wrote:
> On Wed, May 16, 2018 at 06:13:17PM +0300, Michael S. Tsirkin wrote:
> > On Wed, May 16, 2018 at 05:04:41PM +0200, Paolo Bonzini wrote:
> > > On 16/05/2018 16:22, Michael S. Tsirkin wrote:
> > > >> 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.
> > > >
> > > > But when would you want to use the two independently?
> > > 
> > > 1) For testing
> > > 
> > > 2) When some of your QEMUs are too old to support kvm-hint-dedicated=on,
> > > you may still want to use -realtime dedicated-cpus=on to get better
> > > performance on the new one.
> > > 
> > > Paolo
> > 
> > For the second purpose, can't we handle this using machine types?
> 
> Machine-type compatibility code deals with defaults when options
> are omitted, not for making the meaning of explicit options
> depend on the machine-type.
> 
> e.g. having "-machine pc-q35-2.11 -cpu ...,+kvm-hint-dedicated=on"
> not expose the CPUID bit that was explicitly requested in the
> command-line would be a bad idea.

Why? We have machine type affecting guest visible device behaviours
for years.

> -- 
> Eduardo