Re: [Qemu-devel] [RFC PATCH v3 0/9] vfio: capability chains, sparse mmap, device specific regions, IGD support

[Tian, Kevin]

> From: Alex Williamson
> Sent: Wednesday, February 17, 2016 5:32 AM
> 
> v3:

Alex, thanks a lot for your enhancement, which should make future
vgpu work easier!

> 
> Quite a bit of restructuring, functional differences include exposing
> another fw_cfg file to indicate the size of the stolen memory region.
> We really have no need to copy anything into stolen memory, so while
> we tell SeaBIOS about it via a fw_cfg file, the data pointer is NULL
> so it can't be read.
> 
> I'm also now reading the GGMS size from the GMCH register which
> determines the size of the GTT region of stolen memory.  The vBIOS
> is typically only using 1MB, but this is often 2MB in hardware.  I
> also give the user the ability to specify a GMS value for further
> stolen memory.  We default it to zero and it's an experimental option
> so we can remove if it's not useful.  QEMU now does the virtualization
> of the GMCH and BDSM registers, which is was sort of doing before
> anyway, but vfio kernel no longer does anything special for them.
> Getting the GGMS size requires that we know something about the IGD
> version we're using, so code added for that.

I'm not sure how useful keeping stolen memory is. Stolen memory
is not accessed by CPU directly. One is for GTT, but GTT is alias in
PCI bar already. The other is to back as framebuffer for early boot-up,
which is mapped to GTT, so the access actually comes from GPU which
is what RMRR should care about for IOMMU setup.

Thanks
Kevin