linux-kernel - Re: [PATCH v11 1/1] vfio/nvgpu: Add vfio pci variant module for grace hopper

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Date:   Mon, 9 Oct 2023 13:36:12 -0600
From:   Alex Williamson <alex.williamson@...hat.com>
To:     Bagas Sanjaya <bagasdotme@...il.com>
Cc:     <ankita@...dia.com>, <jgg@...dia.com>, <yishaih@...dia.com>,
        <shameerali.kolothum.thodi@...wei.com>, <kevin.tian@...el.com>,
        <aniketa@...dia.com>, <cjia@...dia.com>, <kwankhede@...dia.com>,
        <targupta@...dia.com>, <vsethi@...dia.com>, <acurrid@...dia.com>,
        <apopple@...dia.com>, <jhubbard@...dia.com>, <danw@...dia.com>,
        <anuaggarwal@...dia.com>, <dnigam@...dia.com>, <udhoke@...dia.com>,
        <kvm@...r.kernel.org>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v11 1/1] vfio/nvgpu: Add vfio pci variant module for
 grace hopper

On Sun, 8 Oct 2023 07:06:41 +0700
Bagas Sanjaya <bagasdotme@...il.com> wrote:

> On Sun, Oct 08, 2023 at 01:52:54AM +0530, ankita@...dia.com wrote:
> > PCI BAR are aligned to the power-of-2, but the actual memory on the
> > device may not. A read or write access to the physical address from the
> > last device PFN up to the next power-of-2 aligned physical address
> > results in reading ~0 and dropped writes.
> >   
> 
> Reading garbage or padding in that case?
> 
> Confused...

The coherent memory size is rounded to a power-of-2 to be compliant with
PCI BAR semantics, but reading beyond the implemented size fills the
return buffer with -1 data, as is common on many platforms when reading
from an unimplemented section of the address space.  Thanks,

Alex