linux-kernel - Re: [RFC PATCH 0/4] Use 1st-level for DMA remapping in guest

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Message-ID: <20190925065640.GO28074@xz-x1>
Date:   Wed, 25 Sep 2019 14:56:40 +0800
From:   Peter Xu <peterx@...hat.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     "Tian, Kevin" <kevin.tian@...el.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        Jacob Pan <jacob.jun.pan@...ux.intel.com>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "Kumar, Sanjay K" <sanjay.k.kumar@...el.com>,
        "Sun, Yi Y" <yi.y.sun@...el.com>,
        "iommu@...ts.linux-foundation.org" <iommu@...ts.linux-foundation.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Alex Williamson <alex.williamson@...hat.com>,
        David Woodhouse <dwmw2@...radead.org>
Subject: Re: [RFC PATCH 0/4] Use 1st-level for DMA remapping in guest

On Wed, Sep 25, 2019 at 10:48:32AM +0800, Lu Baolu wrote:
> Hi Kevin,
> 
> On 9/24/19 3:00 PM, Tian, Kevin wrote:
> > > > >       '-----------'
> > > > >       '-----------'
> > > > > 
> > > > > This patch series only aims to achieve the first goal, a.k.a using
> > first goal? then what are other goals? I didn't spot such information.
> > 
> 
> The overall goal is to use IOMMU nested mode to avoid shadow page table
> and VMEXIT when map an gIOVA. This includes below 4 steps (maybe not
> accurate, but you could get the point.)
> 
> 1) GIOVA mappings over 1st-level page table;
> 2) binding vIOMMU 1st level page table to the pIOMMU;
> 3) using pIOMMU second level for GPA->HPA translation;
> 4) enable nested (a.k.a. dual stage) translation in host.
> 
> This patch set aims to achieve 1).

Would it make sense to use 1st level even for bare-metal to replace
the 2nd level?

What I'm thinking is the DPDK apps - they have MMU page table already
there for the huge pages, then if they can use 1st level as the
default device page table then it even does not need to map, because
it can simply bind the process root page table pointer to the 1st
level page root pointer of the device contexts that it uses.

Regards,

-- 
Peter Xu