linux-kernel - Re: [PATCH v2 5/9] x86/desc: add native_[ig]dt

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Date:   Sat, 15 May 2021 08:29:39 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "H. Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2 5/9] x86/desc: add native_[ig]dt_invalidate() to
 <asm/desc.h>

On 5/14/21 6:43 PM, H. Peter Anvin wrote:
> From: "H. Peter Anvin (Intel)" <hpa@...or.com>
> 
> In some places, we want the native forms of descriptor table
> invalidation. Rather than open-coding them, add explicitly native
> functions to invalidate the GDT and IDT.
> 
> Signed-off-by: H. Peter Anvin (Intel) <hpa@...or.com>
> ---
>  arch/x86/include/asm/desc.h | 15 +++++++++++++++
>  1 file changed, 15 insertions(+)
> 
> diff --git a/arch/x86/include/asm/desc.h b/arch/x86/include/asm/desc.h
> index b8429ae50b71..aa366b2bbc41 100644
> --- a/arch/x86/include/asm/desc.h
> +++ b/arch/x86/include/asm/desc.h
> @@ -224,6 +224,21 @@ static inline void store_idt(struct desc_ptr *dtr)
>  	asm volatile("sidt %0":"=m" (*dtr));
>  }
>  
> +static const struct desc_ptr __invalid_gdt_idt_ptr __maybe_unused = {
> +	.address = 0,
> +	.size = 0
> +};

I'm not convinced that putting this in a header is really a great idea.
How about:

> +
> +static inline void native_gdt_invalidate(void)
> +{
        const struct desc_ptr ... = ...;

> +	native_load_gdt(&__invalid_gdt_idt_ptr);
> +}

That should generate two PUSH instructions and may well result in a
smaller binary :)

--Andy