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Re: [PATCH] xen/vpci: zero guest-visible BAR addresses to ensure domU asssigns its own
- To: Jan Beulich <jbeulich@xxxxxxxx>, Ariadne Conill <ariadne@ariadne.space>
- From: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>
- Date: Wed, 25 Feb 2026 21:50:27 -0500
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Steven Noonan <steven@xxxxxxxxx>, <xen-devel@xxxxxxxxxxxxxxxxxxxx>
- Delivery-date: Thu, 26 Feb 2026 02:50:48 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 2/25/26 10:37, Jan Beulich wrote:
> On 25.02.2026 00:12, Ariadne Conill wrote:
>> From: Steven Noonan <steven@xxxxxxxxx>
>>
>> If we just use the host's BAR addresses, the domU might not attempt to
>> reconfigure the BAR ranges and may never try to map them with the IOMMU.
>> Zeroing them ensures the guest kernel knows the BARs are not configured
>> and needs to make its own choices about where to map the BARs.
>
> Yet for this, don't we first need to expose a full topology to the guest,
> i.e. at least a host bridge, and maybe further bridges?
While we eventually do want to expose (a) virtual bridge(s) to vPCI domUs (this
work is currently in development), I don't think it's pre-requisite for the code
change herein: clearly, leaking host BAR addresses to domUs isn't right, and
there's no need to wait to address that.
With that said, the commit title/description don't align well with the code
change. Assuming we want to move the code change forward, for v2 of the patch I
suggest dropping the 2nd half of the title, and reworking the commit description
to focus on describing the code change at hand and less on what the domU might
do.
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