Messages in this thread | | | Date | Thu, 21 May 2020 17:59:17 +0100 | From | Catalin Marinas <> | Subject | Re: [PATCH] arm64/cpufeature: Move BUG_ON() inside get_arm64_ftr_reg() |
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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@arm.com> > > >>>> Cc: Will Deacon <will@kernel.org> > > >>>> Cc: Suzuki K Poulose <suzuki.poulose@arm.com> > > >>>> Cc: Mark Brown <broonie@kernel.org> > > >>>> Cc: linux-arm-kernel@lists.infradead.org > > >>>> Cc: linux-kernel@vger.kernel.org > > >>>> > > >>>> Signed-off-by: Anshuman Khandual <anshuman.khandual@arm.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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