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Re: [PATCH v5 8/9] drivers/char: allow driving the rest of XHCI by a domain while Xen uses DbC
- To: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Fri, 26 Aug 2022 16:48:02 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, George Dunlap <george.dunlap@xxxxxxxxxx>, Julien Grall <julien@xxxxxxx>, Stefano Stabellini <sstabellini@xxxxxxxxxx>, Wei Liu <wl@xxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Fri, 26 Aug 2022 14:48:16 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 22.08.2022 17:27, Marek Marczykowski-Górecki wrote:
> That's possible, because the capability was designed specifically to
> allow separate driver handle it, in parallel to unmodified xhci driver
> (separate set of registers, pretending the port is "disconnected" for
> the main xhci driver etc). It works with Linux dom0, although requires
> an awful hack - re-enabling bus mastering behind dom0's backs.
> Linux driver does similar thing - see
> drivers/usb/early/xhci-dbc.c:xdbc_handle_events().
>
> When controller sharing is enabled in kconfig (option marked as
> experimental), dom0 is allowed to use the controller even if Xen uses it
> for debug console. Additionally, option `dbgp=xhci,share=` is available
> to either prevent even dom0 from using it (`no` value), or allow any
> domain using it (`any` value).
>
> In any case, to avoid Linux messing with the DbC, mark this MMIO area as
> read-only. This might cause issues for Linux's driver (if it tries to
> write something on the same page - like anoter xcap), but makes Xen's
> use safe. In practice, as of Linux 5.18, it seems to work without
> issues.
>
> Signed-off-by: Marek Marczykowski-Górecki <marmarek@xxxxxxxxxxxxxxxxxxxxxx>
Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
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