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From: Michael Kelley <mhklinux@outlook.com>
To: Long Li <longli@microsoft.com>, Haiyang Zhang <haiyangz@microsoft.com>,
	"wei.liu@kernel.org" <wei.liu@kernel.org>, Dexuan Cui <decui@microsoft.com>,
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	"dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>, "hpa@zytor.com"
	<hpa@zytor.com>, "arnd@arndb.de" <arnd@arndb.de>, "tytso@mit.edu"
	<tytso@mit.edu>, jason <jason@zx2c4.com>, "x86@kernel.org" <x86@kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	"linux-hyperv@vger.kernel.org" <linux-hyperv@vger.kernel.org>,
	"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>
Subject: RE: [PATCH v2 1/1] x86/hyperv: Use Hyper-V entropy to seed guest
 random number generator
Thread-Topic: [PATCH v2 1/1] x86/hyperv: Use Hyper-V entropy to seed guest
 random number generator
Thread-Index: AQHacMAZOd50lMXg2E+WGXTGLlHRJ7E1IpWAgAAI74A=
Date: Wed, 13 Mar 2024 05:29:08 +0000
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From: Long Li <longli@microsoft.com> Sent: Tuesday, March 12, 2024 9:51 PM
>=20
> > +void __init ms_hyperv_late_init(void)
> > +{
> > +       struct acpi_table_header *header;
> > +       acpi_status status;
> > +       u8 *randomdata;
> > +       u32 length, i;
> > +
> > +       /*
> > +        * Seed the Linux random number generator with entropy provided=
 by
> > +        * the Hyper-V host in ACPI table OEM0.  It would be nice to do=
 this
> > +        * even earlier in ms_hyperv_init_platform(), but the ACPI subs=
ystem
> > +        * isn't set up at that point. Skip if booted via EFI as generi=
c EFI
> > +        * code has already done some seeding using the EFI RNG protoco=
l.
> > +        */
> > +       if (!IS_ENABLED(CONFIG_ACPI) || efi_enabled(EFI_BOOT))
> > +               return;
> > +
> > +       status =3D acpi_get_table("OEM0", 0, &header);
> > +       if (ACPI_FAILURE(status) || !header)
> > +               return;
>=20
> Should we call acpi_put_table() if header =3D=3D 0?

No.  acpi_get_table() setting header to NULL is equivalent to
returning a failure status, per a comment in the code for
acpi_get_table().  So checking header for NULL is probably
redundant, but it doesn't hurt.

> It will also be helpful doing a
> pr_info() here so user knows that hyper-v random number is not used.

In v1 of the patch, I had such a pr_info(), but Wei Liu recommended
removing it, and I agreed it wasn't really necessary.  See the brief=20
discussion here:

https://lore.kernel.org/linux-hyperv/SN6PR02MB4157B61CA09C0DAF0BB994E1D4212=
@SN6PR02MB4157.namprd02.prod.outlook.com/

Michael