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

From: Andrew Jones
Subject: Re: [PATCH v2 0/2] MTE support for KVM guest
Date: Thu, 10 Sep 2020 07:44:40 +0200

On Wed, Sep 09, 2020 at 06:45:33PM -0700, Richard Henderson wrote:
> On 9/9/20 8:25 AM, Andrew Jones wrote:
> >>  * 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).
> > 
> > Maybe we need a new version of KVM_GET_DIRTY_LOG that also provides
> > the tags for all addresses of each dirty page.
> KVM_GET_DIRTY_LOG just provides one bit per dirty page, no?  Then VMM copies
> the data out from its local address to guest memory.
> There'd be no difference with or without tags, afaik.  It's just about how VMM
> copies the data, with or without tags.

Right, as long as it's fast enough to do

  for_each_dirty_page(page, dirty_log)
    for (i = 0; i < host-page-size/16; i += 16)
      append_tag(LDG(page + i))

to get all the tags for each dirty page. I understood it would be faster
to use LDGM, but we'd need a new ioctl for that. So I was proposing we
just piggyback on a new dirty-log ioctl instead.


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