linux-kernel - Re: [PATCH v17 3/3] vfio/nvgrace-gpu: Add vfio pci variant module for grace hopper

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Message-ID: <20240209110044.0cb6707d.alex.williamson@redhat.com>
Date: Fri, 9 Feb 2024 11:00:44 -0700
From: Alex Williamson <alex.williamson@...hat.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: Ankit Agrawal <ankita@...dia.com>, "Tian, Kevin" <kevin.tian@...el.com>,
 Yishai Hadas <yishaih@...dia.com>, "mst@...hat.com" <mst@...hat.com>,
 "shameerali.kolothum.thodi@...wei.com"
 <shameerali.kolothum.thodi@...wei.com>, "clg@...hat.com" <clg@...hat.com>,
 "oleksandr@...alenko.name" <oleksandr@...alenko.name>, "K V P,
 Satyanarayana" <satyanarayana.k.v.p@...el.com>, "eric.auger@...hat.com"
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 "horms@...nel.org" <horms@...nel.org>, Rahul Rameshbabu
 <rrameshbabu@...dia.com>, Aniket Agashe <aniketa@...dia.com>, Neo Jia
 <cjia@...dia.com>, Kirti Wankhede <kwankhede@...dia.com>, "Tarun Gupta
 (SW-GPU)" <targupta@...dia.com>, Vikram Sethi <vsethi@...dia.com>, Andy
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 Aggarwal (SW-GPU)" <anuaggarwal@...dia.com>, Matt Ochs <mochs@...dia.com>,
 "kvm@...r.kernel.org" <kvm@...r.kernel.org>, "linux-kernel@...r.kernel.org"
 <linux-kernel@...r.kernel.org>, "virtualization@...ts.linux-foundation.org"
 <virtualization@...ts.linux-foundation.org>
Subject: Re: [PATCH v17 3/3] vfio/nvgrace-gpu: Add vfio pci variant module
 for grace hopper

On Fri, 9 Feb 2024 13:19:03 -0400
Jason Gunthorpe <jgg@...dia.com> wrote:

> On Fri, Feb 09, 2024 at 08:55:31AM -0700, Alex Williamson wrote:
> > I think Kevin's point is also relative to this latter scenario, in the
> > L1 instance of the nvgrace-gpu driver the mmap of the usemem BAR is
> > cachable, but in the L2 instance of the driver where we only use the
> > vfio-pci-core ops nothing maintains that cachable mapping.  Is that a
> > problem?  An uncached mapping on top of a cachable mapping is often
> > prone to problems.    
> 
> On these CPUs the ARM architecture won't permit it, the L0 level
> blocks uncachable using FWB and page table attributes. The VM, no
> matter what it does, cannot make the cachable memory uncachable.

Great, thanks,

Alex