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Date:   Mon, 17 Jul 2023 12:35:24 -0600
From:   Alex Williamson <alex.williamson@redhat.com>
To:     Jason Gunthorpe <jgg@nvidia.com>
Cc:     Catalin Marinas <catalin.marinas@arm.com>,
        Benjamin Herrenschmidt <benh@kernel.crashing.org>,
        Marc Zyngier <maz@kernel.org>, ankita@nvidia.com,
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        Lorenzo Pieralisi <lpieralisi@kernel.org>,
        Clint Sbisa <csbisa@amazon.com>, osamaabb@amazon.com
Subject: Re: [PATCH v3 1/6] kvm: determine memory type from VMA
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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,

Alex

> > > This is particularly suited for the case (which used to exist, I don't
> > > know if it still does) where the buffer that wants write combining
> > > reside in the same BAR as registers that otherwise don't.  
> > 
> > IIUC that's still the case for some devices (I think Jason mentioned
> > some Mellanox cards).  
> 
> Right, VFIO will have to allow it page-by-page
> 
> > I think this interface would help KVM when we'll need a cacheable
> > mapping. For WC, we are ok without any VFIO changes.  
> 
> Yes, it may be interesting to map cachable CXL memory as NORMAL_NC
> into userspace for similar reasons.
> 
> Jason
>