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Date:   Fri, 2 Jun 2023 16:17:59 +0000
From:   Wei Liu <wei.liu@kernel.org>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Saurabh Sengar <ssengar@linux.microsoft.com>, kys@microsoft.com,
        haiyangz@microsoft.com, wei.liu@kernel.org, decui@microsoft.com,
        tglx@linutronix.de, mingo@redhat.com, bp@alien8.de,
        dave.hansen@linux.intel.com, x86@kernel.org,
        mikelley@microsoft.com, linux-kernel@vger.kernel.org,
        linux-hyperv@vger.kernel.org, hpa@zytor.com
Subject: Re: [PATCH] x86/hyperv: add noop functions to x86_init mpparse
 functions
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On Fri, Jun 02, 2023 at 08:33:13AM -0700, Dave Hansen wrote:
> On 6/2/23 05:41, Saurabh Sengar wrote:
> > In !ACPI system, there is no way to disable CONFIG_X86_MPPARSE.
> > When CONFIG_X86_MPPARSE is enabled for VTL2, the kernel will
> > scan low memory looking for MP tables. Don't allow this, because
> > low memory is controlled by VTL0 and may contain actual valid
> > tables for VTL0, which can confuse the VTL2 kernel.
> 
> Do you folks have a writeup of this VTL* setup anywhere?  I'm struggling
> to grasp why VTL0 and VTL2 share the same address space and why they
> would get confused by each other's data structures.

Dave, here is some public information about Virtual Trust Level.

https://learn.microsoft.com/en-us/virtualization/hyper-v-on-windows/tlfs/vsm#virtual-trust-level-vtl

I wished it could be more detailed.

With the proper configuration, VTL2 can see memory from VTL0, but not
the other way around.

Saurabh can probably give you more information for this particular setup.

Thanks,
Wei.

> 
> $ grep -r VTL[02] Documentation/
> $
> 
> Either way, this is way better than the #ifdefs.  But the changelog is
> kinda just gibberish to me.