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

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Date:   Fri, 4 Jun 2021 17:57:19 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Jason Gunthorpe <jgg@...dia.com>
Cc:     Alex Williamson <alex.williamson@...hat.com>,
        "Tian, Kevin" <kevin.tian@...el.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 04/06/21 17:50, Jason Gunthorpe wrote:
>> Extending the scenarios where WBINVD is not a nop is not a problem for me.
>> If possible I wouldn't mind keeping the existing kvm-vfio connection via the
>> device, if only because then the decision remains in the VFIO camp (whose
>> judgment I trust more than mine on this kind of issue).
> Really the question to answer is what "security proof" do you want
> before the wbinvd can be enabled

I don't want a security proof myself; I want to trust VFIO to make the 
right judgment and I'm happy to defer to it (via the KVM-VFIO device).

Given how KVM is just a device driver inside Linux, VMs should be a 
slightly more roundabout way to do stuff that is accessible to bare 
metal; not a way to gain extra privilege.

Paolo

>   1) User has access to a device that can issue no-snoop TLPS
>   2) User has access to an IOMMU that can not block no-snoop (today)
>   3) Require CAP_SYS_RAW_IO
>   4) Anyone
> 
> #1 is an improvement because it allows userspace to enable wbinvd and
> no-snoop optimizations based on user choice
> 
> #2 is where we are today and wbinvd effectively becomes a fixed
> platform choice. Userspace has no say
> 
> #3 is "there is a problem, but not so serious, root is powerful
>     enough to override"