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

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Message-ID: <20230522095408.02874498@meshulam.tesarici.cz>
Date:   Mon, 22 May 2023 09:54:08 +0200
From:   Petr Tesařík <petr@...arici.cz>
To:     Marek Marczykowski-Górecki 
        <marmarek@...isiblethingslab.com>
Cc:     Christoph Hellwig <hch@....de>, 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, 19 May 2023 12:10:26 +0200
Marek Marczykowski-Górecki <marmarek@...isiblethingslab.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-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.

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

Petr T

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