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Date: Wed, 17 Apr 2024 11:38:13 +0200
From: Borislav Petkov <bp@alien8.de>
To: "Xin Li (Intel)" <xin@zytor.com>
Cc: linux-kernel@vger.kernel.org, luto@kernel.org, tglx@linutronix.de,
	mingo@redhat.com, dave.hansen@linux.intel.com, x86@kernel.org,
	hpa@zytor.com
Subject: Re: [PATCH v3 1/1] x86/fred: Fix INT80 emulation for FRED
Message-ID: <20240417093813.GBZh-YhSQCXgy3OxFI@fat_crate.local>
References: <20240417063001.3773507-1-xin@zytor.com>
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On Tue, Apr 16, 2024 at 11:30:01PM -0700, Xin Li (Intel) wrote:
> 3) The FRED kernel entry handler does *NOT* dispatch INT instructions,
>    which is of event type EVENT_TYPE_SWINT, so compared with
>    do_int80_emulation(), there is no need to do any user mode check.

What does that mean?

An event handler doesn't dispatch INT insns?

/me is confused.

> +#ifdef CONFIG_X86_FRED
> +/*
> + * A FRED-specific INT80 handler fred_int80_emulation() is required:
> + *
> + * 1) As INT instructions and hardware interrupts are separate event
> + *    types, FRED does not preclude the use of vector 0x80 for external
> + *    interrupts. As a result the FRED setup code does *NOT* reserve
> + *    vector 0x80 and calling int80_is_external() is not merely
> + *    suboptimal but actively incorrect: it could cause a system call
> + *    to be incorrectly ignored.
> + *
> + * 2) fred_int80_emulation(), only called for handling vector 0x80 of
> + *    event type EVENT_TYPE_SWINT, will NEVER be called to handle any
> + *    external interrupt (event type EVENT_TYPE_EXTINT).
> + *
> + * 3) The FRED kernel entry handler does *NOT* dispatch INT instructions,
> + *    which is of event type EVENT_TYPE_SWINT, so compared with
> + *    do_int80_emulation(), there is no need to do any user mode check.
> + *
> + * 4) int80_emulation() does a CLEAR_BRANCH_HISTORY, which is likely
> + *    overkill for new x86 CPU implementations that support FRED.
> + *
> + * 5) int $0x80 is the FAST path for 32-bit system calls under FRED.
> + *
> + * A dedicated FRED INT80 handler duplicates quite a bit of the code in
> + * do_int80_emulation(), but it avoids sprinkling more tests and seems
> + * more readable. Just remember that we can always unify common stuff
> + * later if it turns out that it won't diverge anymore, i.e., after the
> + * FRED code settles.
> + */

And this is talking about duplication above and that text is duplicated
from the commit message. :)

I'll zap it when applying.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette