Re: [Xen-devel] How to create PCH to support those existing driver

["Michael S. Tsirkin" <mst@xxxxxxxxxx>]

On Tue, Aug 19, 2014 at 09:24:03PM +0000, Kay, Allen M wrote:
> > Allen,
> > 
> > Could you reply this?
> 
> Let me summarized what we have discussed and learned so far:
> 
> 1) Future Windows/Linux IGD drivers will be modified to restrain from 
> accessing MCH/PCH devices.  We are prototyping this in Windows driver right 
> now and will pass the same methodology to Linux driver once we have a 
> workable solution.  The goal is removing all MCH/PCH accesses in the IGD 
> driver.
> 
> 2) We want the same solution to work in both native and virtualization 
> environments.  Given most driver developers test their changes only in native 
> environment, doing anything specific for virtualization in the driver will 
> cause frequent breakage for virtualization use cases.
> 
> 3) Back porting this new code to support previous generations of HW will be 
> problematic if not impossible.  Each Windows IGD driver release binary 
> supports two generations of HW.  For example, 15.36 driver supports 
> Broadwell/Haswell, 15.33 driver supports Haswell/IvyBridge, 15.31 driver 
> supports Ivybridge/Sandybridge, etc.   Once the driver is product validated, 
> there is little opportunity to  go back and make high impact changes that 
> might affect stability in native environment.
> 
> 4) I agree there is little reason to do anything that requires driver changes 
> at this point,  unless it is the final desired solution.
> 
> The question is whether/how to support IGD passthrough for previous 
> generations of HW?
> 
> 1) If we want to support SandyBridge/IvyBridge/Haswell/Broadwell, we will 
> need most of the original QEMU patches.  We might be able to do without 
> igd_pci_write().  I have tested QEMU changes without igd_pci_write() on both 
> Haswell/Broadwell and Windows booted without any problems.  This will limit 
> only read operations which should reduce a quite a bit of risk to the host 
> platform.

Excellent. I was thinking about changing host's driver to do the writes
in a safe manner, but if don't need that, all the better.
As a next step, we need to limit read operations to specific set of registers
that we can validate.
We can't just pass through reads blindly to host, pci reads have side-effects
and host chipset isn't protected by the iommu.
Since these are legacy devices and drivers, it should be possible to
enumerate all registers that they need.


> 2) If we want the upstream QEMU only work with future driver version, then 
> most of the IGD passthrough patch is probably not needed - with exception of 
> opregion mapping handlers.  The downside is products that depend on this 
> feature will need to apply private patches separately to re-enable IGD 
> passthrough capability.
> 
> Any advice on how should Tiejun proceed from here?
> 
> Allen

I'm fine with either trying to make existing windows and linux drivers
work, or waiting for future drivers.

To me, quick hacks that need minor changes
in driver but don't avoid poking at MCH/PCH don't seem to have value,
I know I proposed some of these myself but this was before I
realised a cleaner solution is possible.


-- 
MST

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