linux-kernel - Re: [PATCH v5 1/6] iommu: add generic boot option iommu.dma

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Date:   Tue, 16 Apr 2019 16:21:00 +0100
From:   Will Deacon <will.deacon@....com>
To:     Robin Murphy <robin.murphy@....com>
Cc:     John Garry <john.garry@...wei.com>,
        Zhen Lei <thunder.leizhen@...wei.com>,
        Jean-Philippe Brucker <jean-philippe.brucker@....com>,
        Joerg Roedel <joro@...tes.org>,
        Jonathan Corbet <corbet@....net>,
        linux-doc <linux-doc@...r.kernel.org>,
        Sebastian Ott <sebott@...ux.ibm.com>,
        Gerald Schaefer <gerald.schaefer@...ibm.com>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        Tony Luck <tony.luck@...el.com>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        David Woodhouse <dwmw2@...radead.org>,
        iommu <iommu@...ts.linux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-s390 <linux-s390@...r.kernel.org>,
        linuxppc-dev <linuxppc-dev@...ts.ozlabs.org>,
        x86 <x86@...nel.org>, linux-ia64 <linux-ia64@...r.kernel.org>,
        Hanjun Guo <guohanjun@...wei.com>
Subject: Re: [PATCH v5 1/6] iommu: add generic boot option iommu.dma_mode

On Fri, Apr 12, 2019 at 02:11:31PM +0100, Robin Murphy wrote:
> On 12/04/2019 11:26, John Garry wrote:
> > On 09/04/2019 13:53, Zhen Lei wrote:
> > > +static int __init iommu_dma_mode_setup(char *str)
> > > +{
> > > +    if (!str)
> > > +        goto fail;
> > > +
> > > +    if (!strncmp(str, "passthrough", 11))
> > > +        iommu_default_dma_mode = IOMMU_DMA_MODE_PASSTHROUGH;
> > > +    else if (!strncmp(str, "lazy", 4))
> > > +        iommu_default_dma_mode = IOMMU_DMA_MODE_LAZY;
> > > +    else if (!strncmp(str, "strict", 6))
> > > +        iommu_default_dma_mode = IOMMU_DMA_MODE_STRICT;
> > > +    else
> > > +        goto fail;
> > > +
> > > +    pr_info("Force dma mode to be %d\n", iommu_default_dma_mode);
> > 
> > What happens if the cmdline option iommu.dma_mode is passed multiple
> > times? We get mutliple - possibily conflicting - prints, right?
> 
> Indeed; we ended up removing such prints for the existing options here,
> specifically because multiple messages seemed more likely to be confusing
> than useful.
> 
> > And do we need to have backwards compatibility, such that the setting
> > for iommu.strict or iommu.passthrough trumps iommu.dma_mode, regardless
> > of order?
> 
> As above I think it would be preferable to just keep using the existing
> options anyway. The current behaviour works out as:
> 
> iommu.passthrough |      Y	|	  N
> iommu.strict	  |      x	|    Y         N
> ------------------|-------------|---------|--------
> MODE		  | PASSTHROUGH | STRICT  |  LAZY
> 
> which seems intuitive enough that a specific dma_mode option doesn't add
> much value, and would more likely just overcomplicate things for users as
> well as our implementation.

Agreed. We can't remove the existing options, and they do the job perfectly
well so I don't see the need to add more options on top.

Will