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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Fri, 18 May 2018 19:17:43 +0200
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Subject: Re: [Qemu-devel] [PATCH] kvm: rename HINTS_DEDICATED to
 KVM_HINTS_REALTIME
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To: Eduardo Habkost <ehabkost@redhat.com>, "Michael S. Tsirkin" <mst@redhat.com>
Cc: linux-kernel@vger.kernel.org, =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= <rkrcmar@redhat.com>, Jonathan Corbet <corbet@lwn.net>, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org, kvm@vger.kernel.org, linux-doc@vger.kernel.org, qemu-devel@nongnu.org

On 18/05/2018 18:04, Eduardo Habkost wrote:
>> Without mlock you should always use pv spinlocks.
>>
>> Otherwise you risk blocking on a lock taken by
>> a VCPU that is in turn blocked on IO, where the IO
>> is not completing because CPU is being used up
>> spinning.
>
> So the stronger guarantee seems necessary.
> 
> Now what should host userspace do if the user is trying to run an
> existing configuration where the CPUID hint was set but memory is
> not pinned?

As mentioned elsewhere in the thread, there are many ways to pin memory,
and mlock is not always necessary.  However, I agree with Michael in
making the hint provide a stronger guarantee.

Paolo