linux-kernel - Re: [PATCH -next] memregion: Add arch_flush

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Date:   Thu, 8 Sep 2022 14:00:11 +0100
From:   Jonathan Cameron <Jonathan.Cameron@...wei.com>
To:     Dan Williams <dan.j.williams@...el.com>
CC:     Borislav Petkov <bp@...en8.de>,
        Davidlohr Bueso <dave@...olabs.net>, <x86@...nel.org>,
        <nvdimm@...ts.linux.dev>, <linux-cxl@...r.kernel.org>,
        <peterz@...radead.org>, <akpm@...ux-foundation.org>,
        <dave.jiang@...el.com>, <vishal.l.verma@...el.com>,
        <ira.weiny@...el.com>, <a.manzanares@...sung.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH -next] memregion: Add arch_flush_memregion() interface

On Wed, 7 Sep 2022 23:53:31 -0700
Dan Williams <dan.j.williams@...el.com> wrote:

> Borislav Petkov wrote:
> > On Wed, Sep 07, 2022 at 09:52:17AM -0700, Dan Williams wrote:  
> > > To be clear nfit stuff and CXL does run in guests, but they do not
> > > support secure-erase in a guest.
> > > 
> > > However, the QEMU CXL enabling is building the ability to do *guest
> > > physical* address space management, but in that case the driver can be
> > > paravirtualized to realize that it is not managing host-physical address
> > > space and does not need to flush caches. That will need some indicator
> > > to differentiate virtual CXL memory expanders from assigned devices.  
> > 
> > Sounds to me like that check should be improved later to ask
> > whether the kernel is managing host-physical address space, maybe
> > arch_flush_memregion() should check whether the address it is supposed
> > to flush is host-physical and exit early if not...  
> 
> Even though I raised the possibility of guest passthrough of a CXL
> memory expander, I do not think it could work in practice without it
> being a gigantic security nightmare. So it is probably safe to just do
> the hypervisor check and assume that there's no such thing as guest
> management of host physical address space.

Agreed.  Other than occasional convenience of doing driver development
in a VM (they reboot quickly ;) can't see why a product system would need
to pass a CXL type 3 device through and as you say security would be 'interesting'
if it were done.  Might be GPU usecases down the line but I'm doubtful on that
as well.

Jonathan