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Re: [PATCH] xen/vpci: zero guest-visible BAR addresses to ensure domU asssigns its own
- To: Stewart Hildebrand <stewart.hildebrand@xxxxxxx>, Ariadne Conill <ariadne@ariadne.space>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Thu, 26 Feb 2026 08:47:52 +0100
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- Cc: Roger Pau Monné <roger.pau@xxxxxxxxxx>, Steven Noonan <steven@xxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Thu, 26 Feb 2026 07:48:00 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 26.02.2026 03:50, Stewart Hildebrand wrote:
> 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.
That would indeed work for me.
Jan
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