linux-kernel - Re: [PATCH v2 8/8] x86/fpu/xstate: Restore supervisor xstates for __fpu__restore

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Date:   Fri, 28 Feb 2020 10:11:44 -0800
From:   Yu-cheng Yu <yu-cheng.yu@...el.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Tony Luck <tony.luck@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        Rik van Riel <riel@...riel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Fenghua Yu <fenghua.yu@...el.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 8/8] x86/fpu/xstate: Restore supervisor xstates for
 __fpu__restore_sig()

On Fri, 2020-02-28 at 18:22 +0100, Borislav Petkov wrote:
> On Fri, Feb 28, 2020 at 08:20:27AM -0800, Yu-cheng Yu wrote:
> > When XSAVES writes to an xsave buffer, xsave->header.xcomp_bv is set to
> > include only saved components, effectively changing the buffer's format.
> 
> So you want to *save* the supervisor states and xcomp_bv will be set to
> supervisor states only and since we don't care about the user states
> there - they will be loaded later - we're good.

No!

> Or do you have to set xcomp_bv later in order to save the user
> components too and also rearrange the buffer to undo the format change
> above?

That is the case.  If we save only supervisor states, the buffer becomes
smaller and has only supervisor states.

> We have using_compacted_format() and we do conversion from compacted to
> standard buffers - I'm looking at copy_xstate_to_kernel() et al - so it
> shouldn't be impossible. So to repeat Sebastian's question which you
> ignored:
> 
> "How large is this supervisor state at most? I guess saving the AVX512
> state just to get the 2 bytes of the supervisor state at the right spot
> is not really optimal."

I thought Sebastian was saying XSAVES is not optimal.

CET has 16 bytes for ring-3 setting, 24 bytes for ring-0.
Saving supervisor states somewhere else and copying back is not better
either.

> In any case, this performance penalty better be paid only by those
> who are actually using some supervisor states. I haven't looked at
> the CET patchset but I'm assuming you're setting the CET bit in
> xfeatures_mask_all only when the feature is being actually used?

We save supervisor states only when xfeatures_mask_supervisor() is not
zero.

Yu-cheng