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Date:   Thu, 21 May 2020 17:59:17 +0100
From:   Catalin Marinas <catalin.marinas@....com>
To:     Will Deacon <will@...nel.org>
Cc:     Anshuman Khandual <anshuman.khandual@....com>,
        linux-arm-kernel@...ts.infradead.org, mark.rutland@....com,
        Suzuki K Poulose <suzuki.poulose@....com>,
        Mark Brown <broonie@...nel.org>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] arm64/cpufeature: Move BUG_ON() inside
 get_arm64_ftr_reg()

On Thu, May 21, 2020 at 05:22:15PM +0100, Will Deacon wrote:
> On Thu, May 21, 2020 at 08:45:38AM +0530, Anshuman Khandual wrote:
> > On 05/20/2020 11:09 PM, Will Deacon wrote:
> > > On Wed, May 20, 2020 at 04:47:11PM +0100, Catalin Marinas wrote:
> > >> On Wed, May 20, 2020 at 01:20:13PM +0100, Will Deacon wrote:
> > >>> On Wed, May 20, 2020 at 06:52:54AM +0530, Anshuman Khandual wrote:
> > >>>> There is no way to proceed when requested register could not be searched in
> > >>>> arm64_ftr_reg[]. Requesting for a non present register would be an error as
> > >>>> well. Hence lets just BUG_ON() when the search fails in get_arm64_ftr_reg()
> > >>>> rather than checking for return value and doing the same in some individual
> > >>>> callers.
> > >>>>
> > >>>> But there are some callers that dont BUG_ON() upon search failure. It adds
> > >>>> an argument 'failsafe' that provides required switch between callers based
> > >>>> on whether they could proceed or not.
> > >>>>
> > >>>> Cc: Catalin Marinas <catalin.marinas@....com>
> > >>>> Cc: Will Deacon <will@...nel.org>
> > >>>> Cc: Suzuki K Poulose <suzuki.poulose@....com>
> > >>>> Cc: Mark Brown <broonie@...nel.org>
> > >>>> Cc: linux-arm-kernel@...ts.infradead.org
> > >>>> Cc: linux-kernel@...r.kernel.org
> > >>>>
> > >>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@....com>
> > >>>> ---
> > >>>> Applies on next-20200518 that has recent cpufeature changes from Will.
> > >>>>
> > >>>>  arch/arm64/kernel/cpufeature.c | 26 +++++++++++++-------------
> > >>>>  1 file changed, 13 insertions(+), 13 deletions(-)
> > >>>>
> > >>>> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> > >>>> index bc5048f152c1..62767cc540c3 100644
> > >>>> --- a/arch/arm64/kernel/cpufeature.c
> > >>>> +++ b/arch/arm64/kernel/cpufeature.c
> > >>>> @@ -557,7 +557,7 @@ static int search_cmp_ftr_reg(const void *id, const void *regp)
> > >>>>   *         - NULL on failure. It is upto the caller to decide
> > >>>>   *	     the impact of a failure.
> > >>>>   */
> > >>>> -static struct arm64_ftr_reg *get_arm64_ftr_reg(u32 sys_id)
> > >>>> +static struct arm64_ftr_reg *get_arm64_ftr_reg(u32 sys_id, bool failsafe)
> > >>>
> > >>> Generally, I'm not a big fan of boolean arguments because they are really
> > >>> opaque at the callsite. It also seems bogus to me that we don't trust the
> > >>> caller to pass a valid sys_id, but we trust it to get "failsafe" right,
> > >>> which seems to mean "I promise to check the result isn't NULL before
> > >>> dereferencing it."
> > >>>
> > >>> So I don't see how this patch improves anything. I'd actually be more
> > >>> inclined to stick a WARN() in get_arm64_ftr_reg() when it returns NULL and
> > >>> have the callers handle NULL by returning early, getting rid of all the
> > >>> BUG_ONs in here. Sure, the system might end up in a funny state, but we
> > >>> WARN()d about it and tried to keep going (and Linus has some strong opinions
> > >>> on this too).
> > >>
> > >> Such WARN can be triggered by the user via emulate_sys_reg(), so we
> > >> can't really have it in get_arm64_ftr_reg() without a 'failsafe' option.
> > > 
> > > Ah yes, that would be bad. In which case, I don't think the existing code
> > > should change.
> > 
> > The existing code has BUG_ON() in three different callers doing exactly the
> > same thing that can easily be taken care in get_arm64_ftr_reg() itself. As
> > mentioned before an enum variable (as preferred - over a bool) can still
> > preserve the existing behavior for emulate_sys_reg().
> > 
> > IMHO these are very good reasons for us to change the code which will make
> > it cleaner while also removing three redundant BUG_ON() instances. Hence I
> > will request you to please reconsider this proposal.
> 
> Hmm, then how about trying my proposal with the WARN_ON(), but having a
> get_arm64_ftr_reg_nowarn() variant for the user emulation case?

That works for me, get_arm64_ftr_reg() would be a wrapper over the
_nowarn function with the added WARN_ON.

read_sanitised_ftr_reg() would need to return something though. Would
all 0s be ok? I think it works as long as we don't have negative CPUID
fields.

-- 
Catalin

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