linux-kernel - Re: [PATCH] kvm: rename HINTS_DEDICATED to KVM_HINTS

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Message-ID: <9d528020-87f6-a600-9d08-02c88d20b18c@redhat.com>
Date:   Fri, 18 May 2018 19:17:43 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Eduardo Habkost <ehabkost@...hat.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     linux-kernel@...r.kernel.org,
        Radim Krčmář <rkrcmar@...hat.com>,
        Jonathan Corbet <corbet@....net>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        kvm@...r.kernel.org, linux-doc@...r.kernel.org,
        qemu-devel@...gnu.org
Subject: Re: [PATCH] kvm: rename HINTS_DEDICATED to KVM_HINTS_REALTIME

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