netdev - Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel virtual address

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Message-ID: <20190307211506-mutt-send-email-mst@kernel.org>
Date:   Thu, 7 Mar 2019 21:21:03 -0500
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Jerome Glisse <jglisse@...hat.com>
Cc:     Jason Wang <jasowang@...hat.com>, kvm@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, peterx@...hat.com,
        linux-mm@...ck.org, aarcange@...hat.com
Subject: Re: [RFC PATCH V2 5/5] vhost: access vq metadata through kernel
 virtual address

On Thu, Mar 07, 2019 at 02:17:20PM -0500, Jerome Glisse wrote:
> > It's because of all these issues that I preferred just accessing
> > userspace memory and handling faults. Unfortunately there does not
> > appear to exist an API that whitelists a specific driver along the lines
> > of "I checked this code for speculative info leaks, don't add barriers
> > on data path please".
> 
> Maybe it would be better to explore adding such helper then remapping
> page into kernel address space ?

I explored it a bit (see e.g. thread around: "__get_user slower than
get_user") and I can tell you it's not trivial given the issue is around
security.  So in practice it does not seem fair to keep a significant
optimization out of kernel because *maybe* we can do it differently even
better :)

-- 
MST