linux-kernel - Re: [PATCH 2/4] x86: always initialize xen-swiotlb when xen-pcifront is enabling

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Date:   Fri, 19 May 2023 13:49:46 +0100
From:   Andrew Cooper <andrew.cooper3@...rix.com>
To:     Christoph Hellwig <hch@....de>,
        Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>
Cc:     Juergen Gross <jgross@...e.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, Ben Skeggs <bskeggs@...hat.com>,
        Karol Herbst <kherbst@...hat.com>,
        Lyude Paul <lyude@...hat.com>, xen-devel@...ts.xenproject.org,
        iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
        nouveau@...ts.freedesktop.org
Subject: Re: [PATCH 2/4] x86: always initialize xen-swiotlb when xen-pcifront
 is enabling

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