Re: [PATCH 2/4] iommu: Use group ownership to avoid driver attachment

[Robin Murphy]

On 2023-02-15 07:28, Baolu Lu wrote:
> On 2023/2/15 14:56, Tian, Kevin wrote:
> > > From: Baolu Lu<baolu.lu@xxxxxxxxxxxxxxx>
> > > Sent: Wednesday, February 15, 2023 1:51 PM
> > >
> > > On 2/13/23 10:19 PM, Jason Gunthorpe wrote:
> > > > On Mon, Feb 13, 2023 at 03:49:39PM +0800, Lu Baolu wrote:
> > > > > @@ -2992,6 +2987,14 @@ static ssize_t iommu_group_store_type(struct
> > > iommu_group *group,
> > > > >        else
> > > > >            return -EINVAL;
> > > > >
> > > > > +    if (req_type != IOMMU_DOMAIN_DMA_FQ ||
> > > > > +        group->default_domain->type != IOMMU_DOMAIN_DMA) {
> > > > > +        ret = iommu_group_claim_dma_owner(group, (void *)buf);
> > > > > +        if (ret)
> > > > > +            return ret;
> > > > > +        group_owner_claimed = true;
> > > > > +    }
> > > > I don't get it, this should be done unconditionally. If we couldn't
> > > > take ownership then we simply can't progress.
> > > The existing code allows the user to switch the default domain from
> > > strict to lazy invalidation mode. The default domain is not changed,
> > > hence it should be seamless and transparent to the device driver.
> > Is there real usage relying on this transition for a bound device?
> >
> > In concept strict->lazy transition implies relaxed DMA security. It's 
> > hard
> > to think of a motivation of doing so while the device might be doing
> > in-fly DMAs.
> >
> > Presumably such perf/security tradeoff should be planned way before
> > binding device/driver together.
> >
> > btw if strict->lazy is allowed why lazy->strict is prohibited?
>
> We all know, strict vs. lazy is a tradeoff between performance and
> security.
>
> strict -> lazy: driver works in secure mode. This transition trades off
> security for better performance.
>
> lazy->strict: The driver is already working in non-safety mode. This
> transition only results in worse performance. It makes no sense. If user
> want to put the driver in a secure mode, they need to unbind the driver,
> reset the device and do the lazy->strict transition.
>
> Robin might have better insights.

Yes, this was added for a definite use-case in ChromeOS, where 
strict->lazy needs to support being done "live" since the device in 
question is the storage controller for the already-mounted root 
filesystem. Your reasoning seems to match what I summarised in the 
original commit message :)

Thanks,
Robin.