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

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Message-ID: <ZMK/oN6EUdQnKd6i@Asurada-Nvidia>
Date:   Thu, 27 Jul 2023 12:04:00 -0700
From:   Nicolin Chen <nicolinc@...dia.com>
To:     Jason Gunthorpe <jgg@...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 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?