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Date:   Thu, 10 Sep 2020 15:39:19 +0200
From:   Andrew Jones <drjones@redhat.com>
To:     "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc:     Richard Henderson <richard.henderson@linaro.org>,
        Steven Price <steven.price@arm.com>,
        Peter Maydell <Peter.Maydell@arm.com>,
        Juan Quintela <quintela@redhat.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        qemu-devel@nongnu.org, kvmarm@lists.cs.columbia.edu,
        linux-arm-kernel@lists.infradead.org,
        Marc Zyngier <maz@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Will Deacon <will@kernel.org>,
        Dave Martin <Dave.Martin@arm.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 0/2] MTE support for KVM guest
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On Thu, Sep 10, 2020 at 02:27:48PM +0100, Dr. David Alan Gilbert wrote:
> * Andrew Jones (drjones@redhat.com) wrote:
> > 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.
> 
> That feels a bad idea to me; there's a couple of different ways dirty
> page checking work; lets keep extracting the tags separate.
>

It's sounding like it was a premature optimization anyway. We don't yet
know if an ioctl for LDGM is worth it. Looping over LDG may work fine.

Thanks,
drew