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 <[email protected]> > >>>> Cc: Will Deacon <[email protected]> > >>>> Cc: Suzuki K Poulose <[email protected]> > >>>> Cc: Mark Brown <[email protected]> > >>>> Cc: [email protected] > >>>> Cc: [email protected] > >>>> > >>>> Signed-off-by: Anshuman Khandual <[email protected]> > >>>> --- > >>>> 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? Will
