Re: [Qemu-devel] HPET configuration in Seabios (was: Re: windows workload: many ept_violation and mmio exits)

[Kevin O'Connor]

On Sun, Aug 28, 2011 at 10:42:49PM +0200, Jan Kiszka wrote:
> On 2011-08-28 20:54, Alexander Graf wrote:
> > 
> > On 28.08.2011, at 02:42, Avi Kivity wrote:
> > 
> >> On 08/26/2011 08:32 AM, ya su wrote:
> >>> hi,Avi:
> >>>
> >>>     I met the same problem, tons of hpet vm_exits(vector 209, fault
> >>> address is in the guest vm's hpet mmio range), even I disable hpet
> >>> device in win7 guest vm, it still produce a larget amount of vm_exits
> >>> when trace-cmd ;  I add -no-hpet to start the vm, it still has HPET
> >>> device inside VM.
> >>>
> >>>     Does that means the HPET device in VM does not depend on the
> >>> emulated hpet device in qemu-kvm? Is there any way to disable the VM
> >>> HPET device to prevent so many vm_exits?  Thansk.
> >>>
> >>
> >> Looks like a bug to me.
> > 
> > IIRC disabling the HPET device doesn't remove the entry from the DSDT, no? 
> > So the guest OS might still think it's there while nothing responds (read 
> > returns -1).
> 
> Exactly. We have a fw_cfg interface in place for quite a while now
> (though I wonder how the firmware is supposed to tell -no-hpet apart
> from QEMU versions that don't provide this data - both return count =
> 255), but SeaBios still exposes one HPET block at a hard-coded address
> unconditionally.
> 
> There was quite some discussion about the corresponding Seabios patches
> back then but apparently no consensus was found. Re-reading it, I think
> Kevin asked for passing the necessary DSDT fragments from QEMU to the
> firmware instead of using a new, proprietary fw_cfg format. Is that
> still the key requirement for any patch finally fixing this bug?

My preference would be to use the existing ACPI table passing
interface (fw_cfg slot 0x8000) to pass different ACPI tables to
SeaBIOS.

SeaBIOS doesn't currently allow that interface to override tables
SeaBIOS builds itself, but it's a simple change to rectify that.

When this was last proposed, it was raised that the header information
in the ACPI table may then not match the tables that SeaBIOS builds.
I think I proposed at that time that SeaBIOS could use the header of
the first fw_cfg table (or some other fw_cfg interface) to populate
the headers of its table headers.  However, there was no consensus.

Note - the above is in regard to the HPET table.  If the HPET entry in
the DSDT needs to be removed then that's a bigger change.

-Kevin