linux-kernel - Re: [RFC PATCH v3 1/2] iommu: Add capability IOMMU_CAP

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Date:   Sun, 17 Jan 2021 07:30:39 +0200
From:   Leon Romanovsky <leon@...nel.org>
To:     Lu Baolu <baolu.lu@...ux.intel.com>
Cc:     tglx@...utronix.de, ashok.raj@...el.com, kevin.tian@...el.com,
        dave.jiang@...el.com, megha.dey@...el.com, dwmw2@...radead.org,
        alex.williamson@...hat.com, bhelgaas@...gle.com,
        dan.j.williams@...el.com, will@...nel.org, joro@...tes.org,
        dmaengine@...r.kernel.org, eric.auger@...hat.com,
        jacob.jun.pan@...el.com, jgg@...lanox.com, kvm@...r.kernel.org,
        kwankhede@...dia.com, linux-kernel@...r.kernel.org,
        linux-pci@...r.kernel.org, iommu@...ts.linux-foundation.org,
        maz@...nel.org, mona.hossain@...el.com, netanelg@...lanox.com,
        parav@...lanox.com, pbonzini@...hat.com, rafael@...nel.org,
        samuel.ortiz@...el.com, sanjay.k.kumar@...el.com,
        shahafs@...lanox.com, tony.luck@...el.com, vkoul@...nel.org,
        yan.y.zhao@...ux.intel.com, yi.l.liu@...el.com
Subject: Re: [RFC PATCH v3 1/2] iommu: Add capability IOMMU_CAP_VIOMMU

On Sat, Jan 16, 2021 at 04:47:40PM +0800, Lu Baolu wrote:
> Hi Leon,
>
> On 2021/1/16 16:39, Leon Romanovsky wrote:
> > On Sat, Jan 16, 2021 at 09:20:16AM +0800, Lu Baolu wrote:
> > > Hi,
> > >
> > > On 2021/1/15 14:31, Leon Romanovsky wrote:
> > > > On Fri, Jan 15, 2021 at 07:49:47AM +0800, Lu Baolu wrote:
> > > > > Hi Leon,
> > > > >
> > > > > On 1/14/21 9:26 PM, Leon Romanovsky wrote:
> > > > > > On Thu, Jan 14, 2021 at 09:30:02AM +0800, Lu Baolu wrote:
> > > > > > > Some vendor IOMMU drivers are able to declare that it is running in a VM
> > > > > > > context. This is very valuable for the features that only want to be
> > > > > > > supported on bare metal. Add a capability bit so that it could be used.
> > > > > >
> > > > > > And how is it used? Who and how will set it?
> > > > >
> > > > > Use the existing iommu_capable(). I should add more descriptions about
> > > > > who and how to use it.
> > > >
> > > > I want to see the code that sets this capability.
> > >
> > > Currently we have Intel VT-d and the virt-iommu setting this capability.
> > >
> > >   static bool intel_iommu_capable(enum iommu_cap cap)
> > >   {
> > >   	if (cap == IOMMU_CAP_CACHE_COHERENCY)
> > >   		return domain_update_iommu_snooping(NULL) == 1;
> > >   	if (cap == IOMMU_CAP_INTR_REMAP)
> > >   		return irq_remapping_enabled == 1;
> > > +	if (cap == IOMMU_CAP_VIOMMU)
> > > +		return caching_mode_enabled();
> > >
> > >   	return false;
> > >   }
> > >
> > > And,
> > >
> > > +static bool viommu_capable(enum iommu_cap cap)
> > > +{
> > > +	if (cap == IOMMU_CAP_VIOMMU)
> > > +		return true;
> > > +
> > > +	return false;
> > > +}
> >
> > These two functions are reading this cap and not setting.
> > Where can I see code that does "cap = IOMMU_CAP_VIOMMU" and not "=="?
>
> The iommu_capable() is a generic IOMMU interface to query IOMMU
> capabilities. It takes @bus and @cap as input, and calls the callback
> of vendor iommu. If the vendor iommu driver supports the specific
> capability, it returns true. Otherwise, it returns false.
>
> bool iommu_capable(struct bus_type *bus, enum iommu_cap cap)
> {
>         if (!bus->iommu_ops || !bus->iommu_ops->capable)
>                 return false;
>
>         return bus->iommu_ops->capable(cap);
> }
> EXPORT_SYMBOL_GPL(iommu_capable);
>
> In the vendor iommu's callback, it checks the capability and returns a
> value according to its capability, just as showed above.

Ohh, sorry.
I missed "iommu_capable(dev->bus, IOMMU_CAP_VIOMMU)" from second patch.

Thanks

>
> Best regards,
> baolu