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Subject: Re: [PATCH v3 1/6] kvm: determine memory type from VMA
From:   Benjamin Herrenschmidt <benh@kernel.crashing.org>
To:     Alex Williamson <alex.williamson@redhat.com>,
        Jason Gunthorpe <jgg@nvidia.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Marc Zyngier <maz@kernel.org>, ankita@nvidia.com,
        naoya.horiguchi@nec.com, oliver.upton@linux.dev,
        aniketa@nvidia.com, cjia@nvidia.com, kwankhede@nvidia.com,
        targupta@nvidia.com, vsethi@nvidia.com, acurrid@nvidia.com,
        apopple@nvidia.com, jhubbard@nvidia.com, danw@nvidia.com,
        kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-arm-kernel@lists.infradead.org, linux-mm@kvack.org,
        Lorenzo Pieralisi <lpieralisi@kernel.org>,
        Clint Sbisa <csbisa@amazon.com>, osamaabb@amazon.com
Date:   Tue, 25 Jul 2023 16:18:48 +1000
In-Reply-To: <20230717123524.0cefedd0.alex.williamson@redhat.com>
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On Mon, 2023-07-17 at 12:35 -0600, Alex Williamson wrote:
> On Sun, 16 Jul 2023 19:30:23 -0300
> Jason Gunthorpe <jgg@nvidia.com> wrote:
> 
> > On Sun, Jul 16, 2023 at 08:09:02AM -0700, Catalin Marinas wrote:
> > 
> > > In terms of security for arm64 at least, Device vs Normal NC (or nc vs
> > > wc in Linux terminology) doesn't make much difference with the former
> > > occasionally being worse. The kernel would probably trust the DPDK code
> > > if it allows direct device access.  
> > 
> > RDMA and DRM already allow device drivers to map WC to userspace on
> > demand, we expect the platform to support this.
> > 
> > > > So the userspace component needs to be responsible for selecting the
> > > > mapping, the same way using the PCI sysfs resource files today allows
> > > > to do that by selecting the _wc variant.  
> > > 
> > > I guess the sysfs interface is just trying to work around the VFIO
> > > limitations.  
> > 
> > I think just nobody has ever asked for VFIO WC support. The main
> > non-VM user is DPDK and none of the NIC drivers have wanted this (DPDK
> > applications areis more of throughput than latency focused typically)
> 
> Yes, QEMU can't know whether the device or driver want a WC BAR
> mapping, so we've left it for KVM manipulation relative to VM use
> cases.  Nobody has followed through with a complete proposal to enable
> it otherwise for direct userspace driver access, but I don't think
> there's opposition to providing such a thing.  Thanks,

Ok, this is really backburner work for me but I'll try to cook up a POC
patch in the near (hopefully) future along the lines of the subregions
I proposed and we can discuss from there.

Cheers,
Ben.