linux-kernel - Re: XSAVE / RDPKRU on Intel 11th Gen Core CPUs

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Message-ID: <CADyq12yrcJkxALo4muEnR3Nm2yyZAvtabN3RY4fm1DSqtNTF0A@mail.gmail.com>
Date:   Tue, 9 Nov 2021 09:14:16 -0500
From:   Brian Geffon <bgeffon@...gle.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Guenter Roeck <groeck@...gle.com>,
        Borislav Petkov <bp@...e.de>,
        Andy Lutomirski <luto@...nel.org>, stable@...r.kernel.org,
        "the arch/x86 maintainers" <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: XSAVE / RDPKRU on Intel 11th Gen Core CPUs

On Tue, Nov 9, 2021 at 8:43 AM Brian Geffon <bgeffon@...gle.com> wrote:

> What's interesting is that writing to init_pkru fails with -EINVAL for me,
> and I've traced it down to get_xsave_addr() returning NULL on the following
> check:
>
>   /*
>   * This assumes the last 'xsave*' instruction to
>   * have requested that 'xfeature_nr' be saved.
>   * If it did not, we might be seeing and old value
>   * of the field in the buffer.
>   *
>   * This can happen because the last 'xsave' did not
>   * request that this feature be saved (unlikely)
>   * or because the "init optimization" caused it
>   * to not be saved.
>   */
>   if (!(xsave->header.xfeatures & BIT_ULL(xfeature_nr)))
>      return NULL;
>

Sorry, I should have probably also shared the value of xfeatures at
this point is 0x3: which appears to be: (X86_FEATURE_FPU |
X86_FEATURE_XMM)

Brian