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Date:   Fri, 19 May 2023 14:41:18 +0200
From:   Christoph Hellwig <hch@lst.de>
To:     Marek =?iso-8859-1?Q?Marczykowski-G=F3recki?= 
        <marmarek@invisiblethingslab.com>
Cc:     Christoph Hellwig <hch@lst.de>, Juergen Gross <jgross@suse.com>,
        Stefano Stabellini <sstabellini@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>, Ben Skeggs <bskeggs@redhat.com>,
        Karol Herbst <kherbst@redhat.com>,
        Lyude Paul <lyude@redhat.com>, xen-devel@lists.xenproject.org,
        iommu@lists.linux.dev, linux-kernel@vger.kernel.org,
        nouveau@lists.freedesktop.org
Subject: Re: [PATCH 2/4] x86: always initialize xen-swiotlb when
 xen-pcifront is enabling
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On Fri, May 19, 2023 at 12:10:26PM +0200, Marek Marczykowski-G?recki wrote:
> While I would say PCI passthrough is not very common for PV guests, can
> the decision about xen-swiotlb be delayed until you can enumerate
> xenstore to check if there are any PCI devices connected (and not
> allocate xen-swiotlb by default if there are none)? This would
> still not cover the hotplug case (in which case, you'd need to force it
> with a cmdline), but at least you wouldn't loose much memory just
> because one of your VMs may use PCI passthrough (so, you have it enabled
> in your kernel).

How early can we query xenstore?  We'd need to do this before setting
up DMA for any device.

The alternative would be to finally merge swiotlb-xen into swiotlb, in
which case we might be able to do this later.  Let me see what I can
do there.