linux-kernel - RE: [patch] x86/realmode: Make stack lock work in trampoline

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Message-ID: <67f939e112cc4578833ca74123bee402@AcuMS.aculab.com>
Date:   Sat, 10 Jun 2023 19:50:58 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Andrew Cooper' <andrew.cooper3@...rix.com>,
        Yunhong Jiang <yunhong.jiang@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>
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        LKML <linux-kernel@...r.kernel.org>,
        "x86@...nel.org" <x86@...nel.org>,
        "David Woodhouse" <dwmw2@...radead.org>,
        Brian Gerst <brgerst@...il.com>,
        "Arjan van de Veen" <arjan@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        "Paul McKenney" <paulmck@...nel.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        "Sean Christopherson" <seanjc@...gle.com>,
        Oleksandr Natalenko <oleksandr@...alenko.name>,
        Paul Menzel <pmenzel@...gen.mpg.de>,
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        Sabin Rapan <sabrapan@...zon.com>,
        "Michael Kelley (LINUX)" <mikelley@...rosoft.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>
Subject: RE: [patch] x86/realmode: Make stack lock work in trampoline_compat()

From: Andrew Cooper
> Sent: 09 June 2023 00:58
> 
...
> The important point is the l suffix on btsl, which forces it to be long
> (32bit) irrespective of the default operand size.

Does that matter at all?
The 'bit' opcodes (I'm sure 'bts' is 'bit test and set') take
a bit offset from the base address.
This accesses the same bit regardless of the operand size.

The one real issue is that a byte operand will only lock the one byte.
This might be problematic if non-bit locked accesses are also used.
Although it would need to be rather obscure use.
(This may be one of them...)

The only other problem is that btsl always does a locked 32bit
access. If the base address is misaligned this is a misaligned
locked access - problematic if it crosses a cache line boundary.

	David

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