Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Message-ID: <96285ed154dbb92686ca0068e21f5e0500bb1ce7.camel@linux.intel.com>
Subject: Re: [RFC PATCH 2/4] kvm: Add host side support for free memory hints
From:   Alexander Duyck <alexander.h.duyck@linux.intel.com>
To:     "Michael S. Tsirkin" <mst@redhat.com>,
        Alexander Duyck <alexander.duyck@gmail.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org, rkrcmar@redhat.com, x86@kernel.org,
        mingo@redhat.com, bp@alien8.de, hpa@zytor.com, pbonzini@redhat.com,
        tglx@linutronix.de, akpm@linux-foundation.org
Date:   Mon, 11 Feb 2019 09:34:25 -0800
In-Reply-To: <20190209194108-mutt-send-email-mst@kernel.org>
References: <20190204181118.12095.38300.stgit@localhost.localdomain>
         <20190204181546.12095.81356.stgit@localhost.localdomain>
         <20190209194108-mutt-send-email-mst@kernel.org>
Content-Type: text/plain; charset="UTF-8"
Mime-Version: 1.0
Content-Transfer-Encoding: 7bit
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Sat, 2019-02-09 at 19:44 -0500, Michael S. Tsirkin wrote:
> On Mon, Feb 04, 2019 at 10:15:46AM -0800, Alexander Duyck wrote:
> > From: Alexander Duyck <alexander.h.duyck@linux.intel.com>
> > 
> > Add the host side of the KVM memory hinting support. With this we expose a
> > feature bit indicating that the host will pass the messages along to the
> > new madvise function.
> > 
> > This functionality is mutually exclusive with device assignment. If a
> > device is assigned we will disable the functionality as it could lead to a
> > potential memory corruption if a device writes to a page after KVM has
> > flagged it as not being used.
> 
> I really dislike this kind of tie-in.
> 
> Yes right now assignment is not smart enough but generally
> you can protect the unused page in the IOMMU and that's it,
> it's safe.
> 
> So the policy should not leak into host/guest interface.
> Instead it is better to just keep the pages pinned and
> ignore the hint for now.

Okay, I can do that. It also gives me a means of benchmarking just the
hypercall cost versus the extra page faults and zeroing.