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 12:10:26 +0200
From:   Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>
To:     Christoph Hellwig <hch@....de>
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 Fri, May 19, 2023 at 06:04:05AM +0200, Christoph Hellwig wrote:
> On Thu, May 18, 2023 at 08:18:39PM +0200, Marek Marczykowski-Górecki wrote:
> > On Thu, May 18, 2023 at 03:42:51PM +0200, Christoph Hellwig wrote:
> > > Remove the dangerous late initialization of xen-swiotlb in
> > > pci_xen_swiotlb_init_late and instead just always initialize
> > > xen-swiotlb in the boot code if CONFIG_XEN_PCIDEV_FRONTEND is enabled.
> > > 
> > > Signed-off-by: Christoph Hellwig <hch@....de>
> > 
> > Doesn't it mean all the PV guests will basically waste 64MB of RAM
> > by default each if they don't really have PCI devices?
> 
> If CONFIG_XEN_PCIDEV_FRONTEND is enabled, and the kernel's isn't booted
> with swiotlb=noforce, yes.

That's "a bit" unfortunate, since that might be significant part of the
VM memory, or if you have a lot of VMs, a significant part of the host
memory - it quickly adds up.
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).
Please remember that guest kernel is not always under full control of
the host admin, so making guests loose 64MB of RAM always, in default
setup isn't good for customers of such VMs...

-- 
Best Regards,
Marek Marczykowski-Górecki
Invisible Things Lab

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