Re: [PATCH v12 05/28] riscv: usercfi state for task and save/restore of CSR_SSP on trap entry/exit

[Deepak Gupta <debug@rivosinc.com>]

On Thu, Apr 24, 2025 at 01:52:43PM +0200, Radim Krčmář wrote:
>2025-04-23T17:00:29-07:00, Deepak Gupta <debug@rivosinc.com>:
>> On Thu, Apr 10, 2025 at 01:04:39PM +0200, Radim Krčmář wrote:
>>>2025-03-14T14:39:24-07:00, Deepak Gupta <debug@rivosinc.com>:
>>>> diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h
>>>> @@ -62,6 +62,9 @@ struct thread_info {
>>>>  	long			user_sp;	/* User stack pointer */
>>>>  	int			cpu;
>>>>  	unsigned long		syscall_work;	/* SYSCALL_WORK_ flags */
>>>> +#ifdef CONFIG_RISCV_USER_CFI
>>>> +	struct cfi_status	user_cfi_state;
>>>> +#endif
>>>
>>>I don't think it makes sense to put all the data in thread_info.
>>>kernel_ssp and user_ssp is more than enough and the rest can comfortably
>>>live elsewhere in task_struct.
>>>
>>>thread_info is supposed to be as small as possible -- just spanning
>>>multiple cache-lines could be noticeable.
>>
>> I can change it to only include only `user_ssp`, base and size.
>
>No need for base and size either -- we don't touch that in the common
>exception code.

got it.

>
>> But before we go there, see below:
>>
>> $ pahole -C thread_info kbuild/vmlinux
>> struct thread_info {
>>          long unsigned int          flags;                /*     0     8 */
>>          int                        preempt_count;        /*     8     4 */
>>
>>          /* XXX 4 bytes hole, try to pack */
>>
>>          long int                   kernel_sp;            /*    16     8 */
>>          long int                   user_sp;              /*    24     8 */
>>          int                        cpu;                  /*    32     4 */
>>
>>          /* XXX 4 bytes hole, try to pack */
>>
>>          long unsigned int          syscall_work;         /*    40     8 */
>>          struct cfi_status          user_cfi_state;       /*    48    32 */
>>          /* --- cacheline 1 boundary (64 bytes) was 16 bytes ago --- */
>>          long unsigned int          a0;                   /*    80     8 */
>>          long unsigned int          a1;                   /*    88     8 */
>>          long unsigned int          a2;                   /*    96     8 */
>>
>>          /* size: 104, cachelines: 2, members: 10 */
>>          /* sum members: 96, holes: 2, sum holes: 8 */
>>          /* last cacheline: 40 bytes */
>> };
>>
>> If we were to remove entire `cfi_status`, it would still be 72 bytes (88 bytes
>> if shadow call stack were enabled) and already spans across two cachelines.
>
>It has only 64 bytes of data without shadow call stack, but it wasted 8
>bytes on the holes.
>a2 is somewhat an outlier that is not used most exception paths and
>excluding it makes everything fit nicely even now.

But we can't exclude shadow call stack. It'll lead to increased size if that
config is selected. A solution has to work for all the cases and not half
hearted effort.

>
>> if shadow call stack were enabled) and already spans across two cachelines. I
>> did see the comment above that it should fit inside a cacheline. Although I
>> assumed its stale comment given that it already spans across cacheline and I
>> didn't see any special mention in commit messages of changes which grew this
>> structure above one cacheline. So I assumed this was a stale comment.
>>
>> On the other hand, whenever enable/lock bits are checked, there is a high
>> likelyhood that user_ssp and other fields are going to be accessed and
>> thus it actually might be helpful to have it all in one cacheline during
>> runtime.
>
>Yes, although accessing enable/lock bits will be relatively rare.
>It seems better to have the overhead during thread setup, rather than on
>every trap.
>
>> So I am not sure if its helpful sticking to the comment which already is stale.
>
>We could fix the holes and also use sp instead of a0 in the
>new_vmalloc_check, so everything would fit better.
>
>We are really close to fitting into a single cache-line, so I'd prefer
>if shadow stack only filled thread_info with data that is used very
>often in the exception handling code.

I don't get what's the big deal if it results in two cachelines. We can
(re)organize data structure in a way the most frequently accessed members are
together in a single cacheline. We just need to find those members.

In the hot path of exception handling, I see accesses to pt_regs on stack as
well. These are definitley different cacheline than thread_info.

I understand the argument of one member field crossing into two cachelines can
have undesired perf effects. I do not understand reasoning that thread_info
exactly has to fit inside one cacheline.

If this was always supposed to fit in a single cacheline, clearly this
invariant isn't/wasn't maintained as changes trickled in. I would like to see
what maintainers have to say or someone who did data analysis on this.


>
>I think we could do without user_sp in thread_info as well, so there are
>other packing options.

Sure, probably somewhere in task_struct. But fact of the matter is that it has
to be saved/restore during exception entry/exit. But then load/store to
task_struct is essentially a different cachline. Not sure what we will achieve
here?

>
>Btw. could ssp be added to pt_regs?

I had that earlier. It breaks user abi. And it was a no go.

>
>Thanks.