linux-kernel - Re: Lazy FPU restoration / moving kernel_fpu

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Date:   Fri, 15 Jun 2018 11:41:44 -0700
From:   Dave Hansen <dave.hansen@...ux.intel.com>
To:     Andy Lutomirski <luto@...nel.org>,
        "Jason A. Donenfeld" <Jason@...c4.com>,
        Rik van Riel <riel@...riel.com>
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>
Subject: Re: Lazy FPU restoration / moving kernel_fpu_end() to context switch

On 06/15/2018 11:31 AM, Andy Lutomirski wrote:
> Using WRPKRU is easy, but,
> unless we do something very clever, actually finding PKRU in the
> in-memory fpstate image may be slightly nontrivial.

Why?

It's at a constant offset during any one boot, for sure.  XSAVEC/XSAVES
can move it around, but only if you change the Requested Feature BitMap
(RFBM) that we pass to XSAVES or change the control register that
enables XSAVE states (XCR0).

We don't change XCR0 after boot, and RFBM is hard-coded to -1 as far as
I remember.