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

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Message-ID: <20200910132748.GF2814@work-vm>
Date:   Thu, 10 Sep 2020 14:27:48 +0100
From:   "Dr. David Alan Gilbert" <dgilbert@...hat.com>
To:     Andrew Jones <drjones@...hat.com>
Cc:     Richard Henderson <richard.henderson@...aro.org>,
        Steven Price <steven.price@....com>,
        Peter Maydell <Peter.Maydell@....com>,
        Juan Quintela <quintela@...hat.com>,
        Catalin Marinas <catalin.marinas@....com>,
        qemu-devel@...gnu.org, kvmarm@...ts.cs.columbia.edu,
        linux-arm-kernel@...ts.infradead.org,
        Marc Zyngier <maz@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Will Deacon <will@...nel.org>,
        Dave Martin <Dave.Martin@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/2] MTE support for KVM guest

* Andrew Jones (drjones@...hat.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.

Dave

> Thanks,
> drew 
-- 
Dr. David Alan Gilbert / dgilbert@...hat.com / Manchester, UK