linux-kernel - Re: [PATCH v8 2/4] iommufd: Add iommufd_access

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Message-ID: <ZMO0Ffl5Z4qJf3VK@nvidia.com>
Date:   Fri, 28 Jul 2023 09:27:01 -0300
From:   Jason Gunthorpe <jgg@...dia.com>
To:     Nicolin Chen <nicolinc@...dia.com>
Cc:     kevin.tian@...el.com, alex.williamson@...hat.com,
        yi.l.liu@...el.com, joro@...tes.org, will@...nel.org,
        robin.murphy@....com, shuah@...nel.org,
        linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
        kvm@...r.kernel.org, linux-kselftest@...r.kernel.org,
        mjrosato@...ux.ibm.com, farman@...ux.ibm.com
Subject: Re: [PATCH v8 2/4] iommufd: Add iommufd_access_replace() API

On Thu, Jul 27, 2023 at 12:04:00PM -0700, Nicolin Chen wrote:
> On Thu, Jul 27, 2023 at 09:03:01AM -0300, Jason Gunthorpe wrote:
> > On Wed, Jul 26, 2023 at 07:59:11PM -0700, Nicolin Chen wrote:
> > 
> > > I just realized that either my v8 or your version calls unmap()
> > > first at the entire cur_ioas. So, there seems to be no point in
> > > doing that fallback re-add routine since the cur_ioas isn't the
> > > same, which I don't feel quite right...
> > 
> > The point is to restore the access back to how it should be on failure
> > so future use of the accesss still does the right thing.
> > 
> > We already have built into this a certain non-atomicity for mdevs,
> > they can see a pin failure during replace if they race an access
> > during this unmap window. This is similar to the real HW iommu's
> > without atomic replace.
> 
> I was concerned about, after the replace, mdev losing all the
> mappings due to the unmap() call, which means the fallback is
> not really a status quo. Do you mean that they could pin those
> lost mappings back?

At this point their shouldn't be mappings in any path with a chance of
success, as I said it is racy already. Not sure we need to fuss about
it futher.

Jason