linux-kernel - RE: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor

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Message-ID: <a159f2e8417746fb88f31a97c6f366ba@AcuMS.aculab.com>
Date:   Wed, 29 Jul 2020 08:36:33 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     "'Madhavan T. Venkataraman'" <madvenka@...ux.microsoft.com>,
        "Andy Lutomirski" <luto@...nel.org>
CC:     "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        "linux-api@...r.kernel.org" <linux-api@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-integrity@...r.kernel.org" <linux-integrity@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-security-module@...r.kernel.org" 
        <linux-security-module@...r.kernel.org>,
        "oleg@...hat.com" <oleg@...hat.com>,
        "x86@...nel.org" <x86@...nel.org>
Subject: RE: [PATCH v1 0/4] [RFC] Implement Trampoline File Descriptor

From: Madhavan T. Venkataraman
> Sent: 28 July 2020 19:52
...
> trampfd faults are instruction faults that go through a different code path than
> the one that calls handle_mm_fault(). Perhaps, it is the handle_mm_fault() that
> is time consuming. Could you clarify?

Given that the expectation is a few instructions in userspace
(eg to pick up the original arguments for a nested call)
the (probable) thousands of clocks taken by entering the
kernel (especially with page table separation) is a massive
delta.

If entering the kernel were cheap no one would have added
the DSO functions for getting the time of day.

	David

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