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Re: [PATCH v2 0/2] MTE support for KVM guest

From: Steven Price
Subject: Re: [PATCH v2 0/2] MTE support for KVM guest
Date: Wed, 9 Sep 2020 10:15:39 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.10.0

On 07/09/2020 16:28, Dr. David Alan Gilbert wrote:
(cc'ing in Eric Auger)

* Steven Price (steven.price@arm.com) wrote:
Arm's Memory Tagging Extension (MTE) adds 4 bits of tag data to every 16
bytes of memory in the system. This along with stashing a tag within the
high bit of virtual addresses allows runtime checking of memory

These patches add support to KVM to enable MTE within a guest. They are
based on Catalin's v9 MTE user-space support series[1].

I'd welcome feedback on the proposed user-kernel ABI. Specifically this
series currently:

  1. Requires the VMM to enable MTE per-VCPU.
  2. Automatically promotes (normal host) memory given to the guest to be
     tag enabled (sets PG_mte_tagged), if any VCPU has MTE enabled. The
     tags are cleared if the memory wasn't previously MTE enabled.
  3. Doesn't provide any new methods for the VMM to access the tags on

(2) and (3) are particularly interesting from the aspect of VM migration.
The guest is able to store/retrieve data in the tags (presumably for the
purpose of tag checking, but architecturally it could be used as just
storage). This means that when migrating a guest the data needs to be
transferred (or saved/restored).

MTE tags are controlled by the same permission model as normal pages
(i.e. a read-only page has read-only tags), so the normal methods of
detecting guest changes to pages can be used. But this would also
require the tags within a page to be migrated at the same time as the
data (since the access control for tags is the same as the normal data
within a page).

(Without understanding anything about your tag system...)

Note that during (normal, non-postcopy) migration the consistency can
be a little loose - until the guest starts running; i.e. you can send
a page that's in themiddle of being modified as long as you make sure
you send it again later so that what the guest sees on the destination
when it runs is consistent; i.e. it would be fine to send your tags
separately to your data and allow them to get a little out of sync, as
long as they caught up before the guest ran.

Yes, you can obviously pro-actively send data early as you as you appropriately deal with any potential changes that the guest might make. I'm not very familiar with exactly how QEMU handles this, so it might not be a problem - I just wanted to point out that we don't have separate access permissions.

(3) may be problematic and I'd welcome input from those familiar with
VMMs. User space cannot access tags unless the memory is mapped with the
PROT_MTE flag. However enabling PROT_MTE will also enable tag checking
for the user space process (assuming the VMM enables tag checking for
the process) and since the tags in memory are controlled by the guest
it's unlikely the VMM would have an appropriately tagged pointer for its
access. This means the VMM would either need to maintain two mappings of
memory (one to access tags, the other to access data) or disable tag
checking during the accesses to data.

Imagine I had a second mapping; what would it look like; how would I get
and restore the tags?

At a very simple level you could do something like:

 normal_mapping = mmap(..., PROT_READ | PROT_WRITE, ..., fd, 0);
 mte_mapping = mmap(..., PROT_READ | PROT_WRITE | PROT_MTE, ..., fd, 0);

 /* access normal mapping as normal */
 normal_mapping[offset] = 0xf00 + normal_mapping[offset + 1];

 /* read tag from mte_mapping */
 uint64_t tag = ldg(&mte_mapping[offset]);

 /* write a new tag value (8)
  * NOTE: tags are stored in the top byte, hence the shift
 stg(0x8ULL << 56, &mte_mapping[offset]);

Where stg() and ldg() are simple wrappers around the new instructions:

        STG x0, [x1]

        LDG x0, [x0]

In terms of migration stream, I guess we have two ways to do this,
either it rides shotgun on the main RAM section pages, transmitting
those few extra bytes whenever we transmit a page, or you have a
separate iteratable device for RAMtags, and it just transmits those.
How you keep the two together is an interesting question.
The shotgun method sounds nasty to avoid putting special cases in the,
already hairy, RAM code.

As you say above it may be possible to simply let the normal RAM and tags get out of sync. E.g. if you send all the normal RAM (marking read-only as you go), then all the tags (not changing the permissions) you will end up with all the pages that have remained read-only (i.e. the guest hasn't modified) being consistent on the destination. Pages that have been written by the guest will be inconsistent, but you were going to have to resend those anyway.

However for post-migration copy you need to copy *both* normal RAM and tags before resuming the guest. You might need special cases for this.

If it's not practical to either disable tag checking in the VMM or
maintain multiple mappings then the alternatives I'm aware of are:

  * Provide a KVM-specific method to extract the tags from guest memory.
    This might also have benefits in terms of providing an easy way to
    read bulk tag data from guest memory (since the LDGM instruction
    isn't available at EL0).
  * Provide support for user space setting the TCMA0 or TCMA1 bits in
    TCR_EL1. These would allow the VMM to generate pointers which are not
    tag checked.

I guess you want the VMM to do as much tagged checked access as possible
on it's own data structures?

Ideally yes, you would want the VMM to have checked accesses for all it's internal data structures because that gives the maximum benefit from MTE.

How do things like virtio work where the qemu or kernel is accessing
guest memory for IO?

Since virtio is effectively emulating a device it should be treated like a device - no tag checking and no tag storage used. This would be the obvious situation where you would use "normal_mapping" as above so tags wouldn't be visible or checked.

Really the VMM is only interested in guest tags for the migration case where it simply needs to preserve them. I don't expect the guest and VMM (or hypervisor) to communicate using tagged memory.


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