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From:   Michael Ellerman <mpe@ellerman.id.au>
To:     Nicholas Piggin <npiggin@gmail.com>,
        Christophe Leroy <christophe.leroy@csgroup.eu>
Cc:     linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH] powerpc/signal32: Force inlining of
 __unsafe_save_user_regs() and save_tm_user_regs_unsafe()
In-Reply-To: <CT5FZBY14ZM4.OYXBJNXQU9A2@wheely>
References: <7e469c8f01860a69c1ada3ca6a5e2aa65f0f74b2.1685955220.git.christophe.leroy@csgroup.eu>
 <CT5FZBY14ZM4.OYXBJNXQU9A2@wheely>
Date:   Fri, 09 Jun 2023 23:06:11 +1000
Message-ID: <874jnghjjw.fsf@mail.lhotse>
MIME-Version: 1.0
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"Nicholas Piggin" <npiggin@gmail.com> writes:
> On Mon Jun 5, 2023 at 6:58 PM AEST, Christophe Leroy wrote:
>> Looking at generated code for handle_signal32() shows calls to a
>> function called __unsafe_save_user_regs.constprop.0 while user access
>> is open.
>>
>> And that __unsafe_save_user_regs.constprop.0 function has two nops at
>> the begining, allowing it to be traced, which is unexpected during
>> user access open window.
>>
>> The solution could be to mark __unsafe_save_user_regs() no trace, but
>> to be on the safe side the most efficient is to flag it __always_inline
>> as already done for function __unsafe_restore_general_regs(). The
>> function is relatively small and only called twice, so the size
>> increase will remain in the noise.
>>
>> Do the same with save_tm_user_regs_unsafe() as it may suffer the
>> same issue.
>
> Could you put a comment so someone doesn't uninline it later?

I think the "unsafe" in the name is probably sufficient to warn people
off, but you never know. Still I'd happily take a patch to add comments :)

> Marking it notrace as well would be sufficient for a comment, if that works.

I nearly did that when applying, but I'm not sure it won't change the
code generation, so I left it as-is.

cheers