linux-kernel - Re: [PATCH 1/2] mm: Add kernel MMU notifier to manage IOTLB/DEVTLB

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Date:   Thu, 14 Dec 2017 19:08:54 -0800
From:   "Raj, Ashok" <ashok.raj@...el.com>
To:     Bob Liu <liubo95@...wei.com>
Cc:     Lu Baolu <baolu.lu@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H . Peter Anvin" <hpa@...or.com>,
        Alex Williamson <alex.williamson@...hat.com>,
        Joerg Roedel <joro@...tes.org>,
        David Woodhouse <dwmw2@...radead.org>,
        Rik van Riel <riel@...hat.com>,
        Michal Hocko <mhocko@...e.com>,
        Dave Jiang <dave.jiang@...el.com>,
        Kees Cook <keescook@...omium.org>, linux-mm@...ck.org,
        x86@...nel.org, linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...el.com>,
        iommu@...ts.linux-foundation.org,
        Vegard Nossum <vegard.nossum@...cle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Huang Ying <ying.huang@...el.com>,
        Matthew Wilcox <willy@...ux.intel.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Ashok Raj <ashok.raj@...el.com>
Subject: Re: [PATCH 1/2] mm: Add kernel MMU notifier to manage IOTLB/DEVTLB

Hi Bob

On Thu, Dec 14, 2017 at 02:07:38PM +0800, Bob Liu wrote:
> On 2017/12/14 11:38, Lu Baolu wrote:

> >>>> We already have an existing MMU notifiers for userspace updates,
> >>>> however we lack the same thing for kernel page table updates. To
> >> Sorry, I didn't get which situation need this notification.
> >> Could you please describe the full scenario?
> > 
> > Okay.
> > 
> > 1. When an SVM capable driver calls intel_svm_bind_mm() with
> >     SVM_FLAG_SUPERVISOR_MODE set in the @flags, the kernel
> >     memory page mappings will be shared between CPUs and
> >     the DMA remapping agent (a.k.a. IOMMU). The page table
> >     entries will also be cached in both IOTLB (located in IOMMU)
> >     and the DEVTLB (located in device).
> > 
> 
> But who/what kind of real device has the requirement to access a kernel VA?
> Looks like SVM_FLAG_SUPERVISOR_MODE is used by nobody?

That's right, there is no inkernel user at the moment, but we certainly see
them coming.

Maybe not the best example :-), but I'm told Lustre and some of the 
modern NIC's also can benefit from SVM in kernel use.

Not a hypothetical use case certainly!


Cheers,
Ashok