Messages in this thread | | | Subject | Re: [PATCH] arm64/cpufeature: Move BUG_ON() inside get_arm64_ftr_reg() | From | Anshuman Khandual <> | Date | Mon, 25 May 2020 05:22:23 +0530 |
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On 05/21/2020 10:29 PM, Catalin Marinas wrote: > 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.
Sure, will do.
> > 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.
Just trying to understand. If get_arm64_ftr_reg() returns NULL, then read_sanitised_ftr_reg() should also return 0 for that non existent register (already been warned in get_arm64_ftr_reg).
@@ -961,8 +972,8 @@ u64 read_sanitised_ftr_reg(u32 id) { struct arm64_ftr_reg *regp = get_arm64_ftr_reg(id);
- /* We shouldn't get a request for an unsupported register */ - BUG_ON(!regp); + if (!regp) + return 0; return regp->sys_val; }
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