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Subject: Re: [virtio-dev] [PATCH] split-ring: Demand that a device must not change descriptor entries


On 11.11.19 18:17, Rob Miller wrote:
what is trying to be solvedÂhere? There is a reason why this is allowed as some vendors update the table when using RX_MERABLE_BUFFERs & F_IN_ORDER features

OK, good to know. Which fields are updated then? Under which conditions (ie. in which states of the device)?

The background is in the link below. My goal was to find a simple way declaring all fields read-only for the device to avoid problems with new implementations. But it looks like it's more complicated...

Jan


Rob Miller
rob.miller@broadcom.com <mailto:rob.miller@broadcom.com>
(919)721-3339


On Mon, Nov 11, 2019 at 12:13 PM Jan Kiszka <jan.kiszka@siemens.com <mailto:jan.kiszka@siemens.com>> wrote:

    From: Jan Kiszka <jan.kiszka@siemens.com
    <mailto:jan.kiszka@siemens.com>>

    So far the spec only indirectly says that a descriptor table entry is
    not modified by a device when processing it. Make this explicit by
    adding it as normative requirement. Existing drivers already depend on
    this.

    See also
    https://lists.oasis-open.org/archives/virtio-dev/201910/msg00057.html.

    Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com
    <mailto:jan.kiszka@siemens.com>>
    ---
     Âsplit-ring.tex | 2 +-
     Â1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/split-ring.tex b/split-ring.tex
    index 123ac9f..bfef62d 100644
    --- a/split-ring.tex
    +++ b/split-ring.tex
    @@ -217,7 +217,7 @@ \subsection{The Virtqueue Descriptor
    Table}\label{sec:Basic Facilities of a Virt
     Â\devicenormative{\subsubsection}{The Virtqueue Descriptor
    Table}{Basic Facilities of a Virtio Device / Virtqueues / The
    Virtqueue Descriptor Table}
     ÂA device MUST NOT write to a device-readable buffer, and a device
    SHOULD NOT
     Âread a device-writable buffer (it MAY do so for debugging or
    diagnostic
    -purposes).
    +purposes). A device MUST NOT write to any descriptor table entry.

     Â\drivernormative{\subsubsection}{The Virtqueue Descriptor
    Table}{Basic Facilities of a Virtio Device / Virtqueues / The
    Virtqueue Descriptor Table}
     ÂDrivers MUST NOT add a descriptor chain longer than $2^{32}$ bytes
    in total;
-- 2.16.4

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--
Siemens AG, Corporate Technology, CT RDA IOT SES-DE
Corporate Competence Center Embedded Linux


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