Re: [PATCH 02/11] riscv: update asm call site in `call_on_irq_stack` to setup correct label

From: Deepak Gupta <debug@rivosinc.com>
To: Sami Tolvanen <samitolvanen@google.com>
Cc: Paul Walmsley <paul.walmsley@sifive.com>,
	Palmer Dabbelt <palmer@dabbelt.com>,
	Albert Ou <aou@eecs.berkeley.edu>,
	Alexandre Ghiti <alex@ghiti.fr>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Nathan Chancellor <nathan@kernel.org>,
	Nicolas Schier <nicolas.schier@linux.dev>,
	Andrew Morton <akpm@linux-foundation.org>,
	David Hildenbrand <david@redhat.com>,
	Lorenzo Stoakes <lorenzo.stoakes@oracle.com>,
	"Liam R. Howlett" <Liam.Howlett@oracle.com>,
	Vlastimil Babka <vbabka@suse.cz>, Mike Rapoport <rppt@kernel.org>,
	Suren Baghdasaryan <surenb@google.com>,
	Michal Hocko <mhocko@suse.com>,
	Nick Desaulniers <nick.desaulniers+lkml@gmail.com>,
	Bill Wendling <morbo@google.com>,
	Monk Chiang <monk.chiang@sifive.com>,
	Kito Cheng <kito.cheng@sifive.com>,
	Justin Stitt <justinstitt@google.com>,
	linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
	linux-kbuild@vger.kernel.org, linux-mm@kvack.org,
	llvm@lists.linux.dev, rick.p.edgecombe@intel.com,
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	ajones@ventanamicro.com, conor.dooley@microchip.com,
	charlie@rivosinc.com, samuel.holland@sifive.com,
	bjorn@rivosinc.com, fweimer@redhat.com, jeffreyalaw@gmail.com,
	heinrich.schuchardt@canonical.com, andrew@sifive.com,
	ved@rivosinc.com
Subject: Re: [PATCH 02/11] riscv: update asm call site in `call_on_irq_stack` to setup correct label
Date: Fri, 25 Jul 2025 09:56:22 -0700	[thread overview]
Message-ID: <aIO3NtHJ1iCoGOs7@debug.ba.rivosinc.com> (raw)
In-Reply-To: <20250725153351.GB1724026@google.com>

On Fri, Jul 25, 2025 at 03:33:51PM +0000, Sami Tolvanen wrote:
>On Thu, Jul 24, 2025 at 04:36:55PM -0700, Deepak Gupta wrote:
>> Call sites written in asm performing indirect call, they need to setup
>> label register (t2/x7) with correct label.
>>
>> Currently first kernel was compiled with `-save-temps` option and
>> normalized function signature string is captured and then placed at the
>> asm callsite.
>>
>> TODO: to write a macro wrapper with toolchain support.
>>
>> Signed-off-by: Deepak Gupta <debug@rivosinc.com>
>> ---
>>  arch/riscv/kernel/entry.S | 1 +
>>  1 file changed, 1 insertion(+)
>>
>> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
>> index 2660faf52232..598e17e800ae 100644
>> --- a/arch/riscv/kernel/entry.S
>> +++ b/arch/riscv/kernel/entry.S
>> @@ -389,6 +389,7 @@ SYM_FUNC_START(call_on_irq_stack)
>>  	load_per_cpu t0, irq_stack_ptr, t1
>>  	li	t1, IRQ_STACK_SIZE
>>  	add	sp, t0, t1
>> +	lui t2, %lpad_hash("FvP7pt_regsE")
>
>Ah, I see. The plan is to hardcode the signatures in assembly code and
>keep them manually in sync with C code. When we implemented KCFI, we
>thought this would become extremely tedious to maintain after a while.

This is extremely tedious and not maintainable and primary reason (among
other secondary ones) to keep this patch series as RFC.

>Do you have any plans to add KCFI-style __kcfi_typeid_<function> symbols
>that would allow labels to be determined from C type signatures instead?

Yes something on similar lines. I have asked toolchain folks (Monk and Kito)
on how best to support this.

>
>Sami