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Re: [PATCH 2/4] x86: always initialize xen-swiotlb when xen-pcifront is enabling
- To: Christoph Hellwig <hch@xxxxxx>, Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
- From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
- Date: Fri, 19 May 2023 13:49:46 +0100
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- Cc: Juergen Gross <jgross@xxxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Thomas Gleixner <tglx@xxxxxxxxxxxxx>, Ingo Molnar <mingo@xxxxxxxxxx>, Borislav Petkov <bp@xxxxxxxxx>, Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>, x86@xxxxxxxxxx, "H. Peter Anvin" <hpa@xxxxxxxxx>, Ben Skeggs <bskeggs@xxxxxxxxxx>, Karol Herbst <kherbst@xxxxxxxxxx>, Lyude Paul <lyude@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx, iommu@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, nouveau@xxxxxxxxxxxxxxxxxxxxx
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 19/05/2023 1:41 pm, Christoph Hellwig wrote:
> On Fri, May 19, 2023 at 12:10:26PM +0200, Marek Marczykowski-Górecki 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.
Not that early. One supported configuration has xenstore not starting
for an indefinite period of time after boot.
> 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.
If that is an option, it would be great to reduce the special-cashing.
~Andrew
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