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Date:   Mon, 16 Oct 2023 12:29:39 -0700
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Subject: Re: [PATCH -tip 3/3] x86/percpu: *NOT FOR MERGE* Implement
 arch_raw_cpu_ptr() with RDGSBASE
From:   Sean Christopherson <seanjc@google.com>
To:     Ingo Molnar <mingo@kernel.org>
Cc:     Uros Bizjak <ubizjak@gmail.com>, x86@kernel.org,
        linux-kernel@vger.kernel.org, Nadav Amit <namit@vmware.com>,
        Andy Lutomirski <luto@kernel.org>,
        Brian Gerst <brgerst@gmail.com>,
        Denys Vlasenko <dvlasenk@redhat.com>,
        "H . Peter Anvin" <hpa@zytor.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Josh Poimboeuf <jpoimboe@redhat.com>,
        Borislav Petkov <bp@alien8.de>
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On Mon, Oct 16, 2023, Ingo Molnar wrote:
> 
> * Uros Bizjak <ubizjak@gmail.com> wrote:
> 
> > Sean says:
> > The instructions are guarded by a CR4 bit, the ucode cost just to check
> > CR4.FSGSBASE is probably non-trivial."
> 
> BTW., a side note regarding the very last paragraph and the CR4 bit ucode 
> cost, given that SMAP is CR4 controlled too:
> 
>  #define X86_CR4_FSGSBASE_BIT    16 /* enable RDWRFSGS support */
>  #define X86_CR4_FSGSBASE        _BITUL(X86_CR4_FSGSBASE_BIT)
>  ...
>  #define X86_CR4_SMAP_BIT        21 /* enable SMAP support */
>  #define X86_CR4_SMAP            _BITUL(X86_CR4_SMAP_BIT)
> 
> And this modifies the behavior of STAC/CLAC, of which we have ~300 
> instances in a defconfig kernel image:
> 
>   kepler:~/tip> objdump -wdr vmlinux | grep -w 'stac' x | wc  -l
>   119
> 
>   kepler:~/tip> objdump -wdr vmlinux | grep -w 'clac' x | wc  -l
>   188
> 
> Are we certain that ucode on modern x86 CPUs check CR4 for every affected 
> instruction?

Not certain at all.  I agree the CR4.FSGSBASE thing could be a complete non-issue
and was just me speculating.

> Could they perhaps use something faster, such as internal microcode-patching
> (is that a thing?), to turn support for certain instructions on/off when the
> relevant CR4 bit is modified, without having to genuinely access CR4 for
> every instruction executed?

I don't know the exact details, but Intel's VMRESUME ucode flow uses some form of
magic to skip consistency checks that aren't relevant for the current (or target)
mode, *without* using conditional branches.  So it's definitely possible/probable
that similar magic is used to expedite things like CPL and CR4 checks.