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Date:   Mon, 22 May 2023 09:54:08 +0200
From:   Petr =?UTF-8?B?VGVzYcWZw61r?= <petr@tesarici.cz>
To:     Marek =?UTF-8?B?TWFyY3p5a293c2tpLUfDs3JlY2tp?= 
        <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, 19 May 2023 12:10:26 +0200
Marek Marczykowski-G=C3=B3recki <marmarek@invisiblethingslab.com> wrote:

> 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-G=C3=B3rec=
ki wrote: =20
> > > On Thu, May 18, 2023 at 03:42:51PM +0200, Christoph Hellwig wrote: =20
> > > > 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 enabl=
ed.
> > > >=20
> > > > Signed-off-by: Christoph Hellwig <hch@lst.de> =20
> > >=20
> > > 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? =20
> >=20
> > If CONFIG_XEN_PCIDEV_FRONTEND is enabled, and the kernel's isn't booted
> > with swiotlb=3Dnoforce, yes. =20
>=20
> 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.

I wonder if dynamic swiotlb allocation might also help with this...

Petr T

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