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Date:   Thu, 5 Oct 2023 15:34:42 -0500
From:   Bjorn Helgaas <helgaas@kernel.org>
To:     Lukas Wunner <lukas@wunner.de>
Cc:     Jonathan Cameron <Jonathan.Cameron@huawei.com>,
        David Howells <dhowells@redhat.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Herbert Xu <herbert@gondor.apana.org.au>,
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        Alex Williamson <alex.williamson@redhat.com>,
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        Dan Williams <dan.j.williams@intel.com>,
        Dave Jiang <dave.jiang@intel.com>,
        "Li, Ming" <ming4.li@intel.com>, Zhi Wang <zhi.a.wang@intel.com>,
        Alistair Francis <alistair.francis@wdc.com>,
        Wilfred Mallawa <wilfred.mallawa@wdc.com>,
        Alexey Kardashevskiy <aik@amd.com>,
        Tom Lendacky <thomas.lendacky@amd.com>,
        Sean Christopherson <seanjc@google.com>,
        Alexander Graf <graf@amazon.com>
Subject: Re: [PATCH 12/12] PCI/CMA: Grant guests exclusive control of
 authentication
Message-ID: <20231005203442.GA790578@bhelgaas>
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On Tue, Oct 03, 2023 at 09:30:58PM +0200, Lukas Wunner wrote:
> On Tue, Oct 03, 2023 at 04:40:48PM +0100, Jonathan Cameron wrote:
> > On Thu, 28 Sep 2023 19:32:42 +0200 Lukas Wunner <lukas@wunner.de> wrote:
> > > At any given time, only a single entity in a physical system may have
> > > an SPDM connection to a device.  That's because the GET_VERSION request
> > > (which begins an authentication sequence) resets "the connection and all
> > > context associated with that connection" (SPDM 1.3.0 margin no 158).
> > > 
> > > Thus, when a device is passed through to a guest and the guest has
> > > authenticated it, a subsequent authentication by the host would reset
> > > the device's CMA-SPDM session behind the guest's back.
> > > 
> > > Prevent by letting the guest claim exclusive CMA ownership of the device
> > > during passthrough.  Refuse CMA reauthentication on the host as long.
> > > After passthrough has concluded, reauthenticate the device on the host.

> Could you (as an English native speaker) comment on the clarity of the
> two sentences "Prevent ... as long." above, as Ilpo objected to them?
> 
> The antecedent of "Prevent" is the undesirable behaviour in the preceding
> sentence (host resets guest's SPDM connection).

I think this means "prevent a reauthentication by the host behind the
guest's back" (which seems to match the first diff hunk), but I agree
it would be helpful to make the connection clearer, e.g.,

  When passing a device through to a guest, mark it as "CMA owned
  exclusively by the guest" for the duration of the passthrough to
  prevent the host from reauthenticating and resetting the device's
  CMA-SPDM session.

> The antecedent of "as long" is "during passthrough" in the preceding
> sentence.

"as long" definitely needs something to connect it with the
passthrough.

Bjorn