Re: [Xen-devel] [PATCH v4 0/7] unsafe big.LITTLE support

[Julien Grall <julien.grall@xxxxxxx>]

On 12/03/18 02:32, Peng Fan wrote:
> On Fri, Mar 09, 2018 at 02:40:25PM +0000, Julien Grall wrote:
> > Hi,
> >
> > On 09/03/18 13:30, Peng Fan wrote:
> > > Hi Julien,
> > > On Fri, Mar 09, 2018 at 10:22:09AM +0000, Julien Grall wrote:
> > > > Hi Peng,
> > > >
> > > > On 09/03/18 09:05, Peng Fan wrote:
> > > > > On Thu, Mar 08, 2018 at 03:13:50PM +0000, Julien Grall wrote:
> > > > > > On 08/03/18 12:43, Peng Fan wrote:
> > > > > > There are a major difference between Dom0 and DomU in your setup.
> > > > > > Dom0 vCPUs are pinned to a specific pCPU, so they can't move around.
> > > > > > For DomU, each vCPU are pinned to a set of pCPUs, so they can move
> > > > > > around.
> > > > > >
> > > > > > But, did you check the DomU has the workaround enabled? I am asking
> > > > > > that because it looks like to me the way to detect the workaround is
> > > > > > based on a device (scu) and not processor. So I am not convinced that
> > > > > > DomU is actually using your workaround.
> > > > > Just checked this. Because xen toolstack create device tree
> > > > > with compatible "compatible = "xen,xenvm-4.10", "xen,xenvm";",
> > > > > but the linux code use "fsl,imx8qm" to detect soc, then call scu
> > > > > to get revision of chip.
> > > > But how does the guest call the scu?
> > > We are doing GPU and display passthrough, also some other IPs passthrough.
> > > we could not totally rely on Dom0 to configure the pinmux, gpio, clk,
> > > relying on dom0 to do that would bring much hack code to our kernel, also
> > > runtime clk set rate in domu could not be done.
> > >
> > > So we expose an interface to domu to directly communicate with SCU(system
> > > control unit).
> > Do you always expect a domain to access the SCU? Even with no
> > passthrough involved?
> only needed when a domain only needs to directly access hardware.
Then your suggested workaround can't work for guest with not device 
passthrough. I would recommend to find a workaround that works for every 
guests.
Cheers,

--
Julien Grall

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