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Re: [PATCH 2/2] Do not access /dev/mem in MSI-X PCI passthrough on Xen
- To: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>, "qemu-devel@xxxxxxxxxx" <qemu-devel@xxxxxxxxxx>
- From: Andrew Cooper <Andrew.Cooper3@xxxxxxxxxx>
- Date: Mon, 14 Nov 2022 19:39:48 +0000
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- Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>, Anthony Perard <anthony.perard@xxxxxxxxxx>, Paul Durrant <paul@xxxxxxx>, "open list:X86 Xen CPUs" <xen-devel@xxxxxxxxxxxxxxxxxxxx>
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- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
- Thread-index: AQHY+F5BmJFZlWQQDUevznp0C5ln9a4+0PkA
- Thread-topic: [PATCH 2/2] Do not access /dev/mem in MSI-X PCI passthrough on Xen
On 14/11/2022 19:20, Marek Marczykowski-Górecki wrote:
> The /dev/mem is used for two purposes:
> - reading PCI_MSIX_ENTRY_CTRL_MASKBIT
> - reading Pending Bit Array (PBA)
>
> The first one was originally done because when Xen did not send all
> vector ctrl writes to the device model, so QEMU might have outdated old
> register value. This has been changed in Xen, so QEMU can now use its
> cached value of the register instead.
>
> The Pending Bit Array (PBA) handling is for the case where it lives on
> the same page as the MSI-X table itself. Xen has been extended to handle
> this case too (as well as other registers that may live on those pages),
> so QEMU handling is not necessary anymore.
>
> Removing /dev/mem access is useful to work within stubdomain, and
> necessary when dom0 kernel runs in lockdown mode.
>
> Signed-off-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
The commit message ought to go further. Using /dev/mem like this is
buggy anyway, because it is trapped and emulated by Xen in whatever
context Qemu is running. Qemu cannot get the actual hardware value, and
even if it could, it would be racy with transient operations needing to
mask the vector.
i.e. it's not just nice-to-remote - it's fixing real corner cases.
~Andrew
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