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Subject: [OASIS Issue Tracker] Commented: (VIRTIO-35) race condition with multi-dword config accesses

    [ http://tools.oasis-open.org/issues/browse/VIRTIO-35?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=35494#action_35494 ] 

Pawel Moll commented on VIRTIO-35:

MMIO change commited to SVN as r68

> race condition with multi-dword config accesses
> -----------------------------------------------
>                 Key: VIRTIO-35
>                 URL: http://tools.oasis-open.org/issues/browse/VIRTIO-35
>             Project: OASIS Virtual I/O Device (VIRTIO) TC
>          Issue Type: Bug
>            Reporter: Michael Tsirkin
> on many architectures, accesses larger than 32 bit can not be atomic.
> Thus access to a device config field of >4 bytes is inherently racy
> in case field can change.
> For example, virtio-blk has
>                 u64 capacity;
> The following race can trigger:
>   driver reads low 32 bit
>   both low and high 32 bit change
>   driver reads high 32 bit
> as a result, capacity observed is composed of
> old low bits and new high bits which does not
> make sense.
> For legacy devices, spec allowed byte by byte access,
> making the race even more common.

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