[Xen-devel] Re: [PATCH][RFC] fix some spinlock issues in vmsi

[Keir Fraser <keir.fraser@xxxxxxxxxxxxx>]

On 06/03/2009 05:31, "Qing He" <qing.he@xxxxxxxxx> wrote:

> A similar problem also exists in msixtbl_write/msixtbl_read. Currently,
> these functions access the mapped msix table using the pointer stored
> in pci_dev, and when pci_dev is removed without the knowledge of the
> guest, pci_dev alongside the fixmaps will be removed and this
> definitely will frustrate these functions. Unfortunately, adding
> spinlocks to msixtbl_{read,write} seems quite expensive, especially for
> multiple guest threads, that's why RCU based lock is used here. For a
> completely secure handling of this, maybe we need a less brute,
> asynchronous pci_remove hypercall or something.

Add the locks, making it correct. Then optimise later as necessary, using
RCU or (perhaps easier) just smaller locks: Per MSI-X or per-pci-dev or
whatever. Throwing in racey code condemns us to subtle races forever more.

 -- Keir



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