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From:   Saurabh Singh Sengar <ssengar@microsoft.com>
To:     "Michael Kelley (LINUX)" <mikelley@microsoft.com>,
        Saurabh Sengar <ssengar@linux.microsoft.com>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "bp@alien8.de" <bp@alien8.de>,
        "dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
        "x86@kernel.org" <x86@kernel.org>, "hpa@zytor.com" <hpa@zytor.com>,
        KY Srinivasan <kys@microsoft.com>,
        Haiyang Zhang <haiyangz@microsoft.com>,
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        Dexuan Cui <decui@microsoft.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>
Subject: RE: [PATCH 2/2] Drivers: hv: Kconfig: Add HYPERV_VTL_MODE
 dependencies
Thread-Topic: [PATCH 2/2] Drivers: hv: Kconfig: Add HYPERV_VTL_MODE
 dependencies
Thread-Index: AQHZhBiTYeqvuruPeE2EOZkq3i12Bq9jbhmAgAY7z6A=
Date:   Wed, 24 May 2023 16:09:46 +0000
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> -----Original Message-----
> From: Michael Kelley (LINUX) <mikelley@microsoft.com>
> Sent: Saturday, May 20, 2023 10:20 PM
> To: Saurabh Sengar <ssengar@linux.microsoft.com>; tglx@linutronix.de;
> mingo@redhat.com; bp@alien8.de; dave.hansen@linux.intel.com;
> x86@kernel.org; hpa@zytor.com; KY Srinivasan <kys@microsoft.com>;
> Haiyang Zhang <haiyangz@microsoft.com>; wei.liu@kernel.org; Dexuan Cui
> <decui@microsoft.com>; linux-kernel@vger.kernel.org; linux-
> hyperv@vger.kernel.org
> Cc: Saurabh Singh Sengar <ssengar@microsoft.com>
> Subject: [EXTERNAL] RE: [PATCH 2/2] Drivers: hv: Kconfig: Add
> HYPERV_VTL_MODE dependencies
>=20
> From: Saurabh Sengar <ssengar@linux.microsoft.com>
> >
> > MTRRs are not per-VTL and are controlled by VTL0, make sure it is
> > disabled for all other VTLs.
>=20
> Clarifying the wording a bit:
>=20
> MTRRs are not per-VTL, but are always controlled by VTL0.   Allow
> building for VTLs other than VTL0 only when MTRR functionality is disable=
d.

Ok

>=20
> >
> > X86_MPPARSE scans low memory for MP tables, which is not required for
> > Hyper-V VTL platforms.
>=20
> Is the above statement saying that MP tables aren't required, so remove t=
he
> code to save space?  Or is there a problem/failure if MP table code
> runs and scans low memory?   *Requiring* that it be disabled, and the
> reference to scanning low memory makes me think it might be the latter, i=
n
> which case I'd suggest this more forceful wording:
>=20
> The MP table code scans low memory, which causes failures in VTLs
> other than VTL0.   Allow building for VTLs other than VTL0 only when
> MP table functionality is disabled.

Ok

>=20
> If you can give slightly more detail about "causes failures" (a panic?
> some other problem?) that would be good too.

When CONFIG_X86_MPPARSE is enabled, the kernel will scan low memory,
looking for MP tables. In Hyper-V VBS setup, lower memory is assigned to
VTL0. This lower memory may contain the actual MPPARSE table for VTL0,
which can confuse the VTLx kernel and cause issues. (x > 0)

>=20
> Michael
>=20
> >
> > Signed-off-by: Saurabh Sengar <ssengar@linux.microsoft.com>
> > ---
> >  drivers/hv/Kconfig | 2 ++
> >  1 file changed, 2 insertions(+)
> >
> > diff --git a/drivers/hv/Kconfig b/drivers/hv/Kconfig index
> > 00242107d62e..c8d443886b9d 100644
> > --- a/drivers/hv/Kconfig
> > +++ b/drivers/hv/Kconfig
> > @@ -16,6 +16,8 @@ config HYPERV
> >  config HYPERV_VTL_MODE
> >  	bool "Enable Linux to boot in VTL context"
> >  	depends on X86_64 && HYPERV
> > +	depends on !MTRR
> > +	depends on !X86_MPPARSE
> >  	default n
> >  	help
> >  	  Virtual Secure Mode (VSM) is a set of hypervisor capabilities and
> > --
> > 2.34.1