linux-kernel - Re: [RFC] /dev/ioasid uAPI proposal

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Date:   Mon, 7 Jun 2021 15:01:04 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Jean-Philippe Brucker <jean-philippe@...aro.org>,
        "Jiang, Dave" <dave.jiang@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        Robin Murphy <robin.murphy@....com>,
        LKML <linux-kernel@...r.kernel.org>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        David Gibson <david@...son.dropbear.id.au>,
        Kirti Wankhede <kwankhede@...dia.com>,
        David Woodhouse <dwmw2@...radead.org>,
        Jason Wang <jasowang@...hat.com>
Subject: Re: [RFC] /dev/ioasid uAPI proposal

On Mon, Jun 07, 2021 at 08:51:42AM +0200, Paolo Bonzini wrote:
> On 07/06/21 05:25, Tian, Kevin wrote:
> > Per Intel SDM wbinvd is a privileged instruction. A process on the
> > host has no privilege to execute it.
> 
> (Half of) the point of the kernel is to do privileged tasks on the
> processes' behalf.  There are good reasons why a process that uses VFIO
> (without KVM) could want to use wbinvd, so VFIO lets them do it with a ioctl
> and adequate checks around the operation.

Yes, exactly.

You cannot write a correct VFIO application for hardware that uses the
no-snoop bit without access to wbinvd.

KVM or not does not matter.

Jason