direct PIO/MMIO access

direct PIO/MMIO access

[Gábor PÉK]

Hi all,

I would like to access the configuration registers of my passthrough
device from my HVM guest under Xen 4.2 via MMIO/PIO directly. Is there
any option for this in the config file of the guest? I know about the
ioports=[''] option for PIO accesses, but is there any way to do the
same with MMIO BAR registers? As far as I know the permissive flag works
only for PV guests. Does the ioports option allow direct access to
config space (0xcf8-0xcfc)? According to in xen/arch/x86/trap.c it is
not allowed.

    /*
     * Port 0xcf8 (CONFIG_ADDRESS) is only visible for DWORD accesses.
     * We never permit direct access to that register.
     */

What about Dom0?

Thank you!
-gabor

Re: direct PIO/MMIO access

[Jan Beulich]

>>> On 15.02.13 at 17:31, Gábor PÉK<pek@crysys.hu> wrote:
> I would like to access the configuration registers of my passthrough
> device from my HVM guest under Xen 4.2 via MMIO/PIO directly. Is there
> any option for this in the config file of the guest? I know about the
> ioports=[''] option for PIO accesses, but is there any way to do the
> same with MMIO BAR registers?

For a PCI device, no extra option should be needed for either
IO ports or MMIO regions.

> As far as I know the permissive flag works
> only for PV guests. Does the ioports option allow direct access to
> config space (0xcf8-0xcfc)? According to in xen/arch/x86/trap.c it is
> not allowed.

Correct, albeit that code doesn't control HVM guests. But if you
gave HVM guests access to these _physical_ ports, they could
at once control all other PCI devices. So I'm sure you don't want
to do that.

>     /*
>      * Port 0xcf8 (CONFIG_ADDRESS) is only visible for DWORD accesses.
>      * We never permit direct access to that register.
>      */
> 
> What about Dom0?

Not even Dom0. This gets emulated in Xen.

Jan

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