Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::3:5 as permitted sender) client-ip=2620:137:e000::3:5;
Date:   Mon, 9 Oct 2023 13:36:12 -0600
From:   Alex Williamson <alex.williamson@redhat.com>
To:     Bagas Sanjaya <bagasdotme@gmail.com>
Cc:     <ankita@nvidia.com>, <jgg@nvidia.com>, <yishaih@nvidia.com>,
        <shameerali.kolothum.thodi@huawei.com>, <kevin.tian@intel.com>,
        <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>,
        <anuaggarwal@nvidia.com>, <dnigam@nvidia.com>, <udhoke@nvidia.com>,
        <kvm@vger.kernel.org>, <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v11 1/1] vfio/nvgpu: Add vfio pci variant module for
 grace hopper
Message-ID: <20231009133612.3fdd86a9.alex.williamson@redhat.com>
In-Reply-To: <ZSHykZ2GgSn0fE_x@debian.me>
References: <20231007202254.30385-1-ankita@nvidia.com>
        <ZSHykZ2GgSn0fE_x@debian.me>
Organization: Red Hat
MIME-Version: 1.0
Content-Type: text/plain; charset=US-ASCII
Content-Transfer-Encoding: 7bit
Precedence: bulk

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

> On Sun, Oct 08, 2023 at 01:52:54AM +0530, ankita@nvidia.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