linux-kernel - Re: [patch V2 09/37] rseq: Introduce struct rseq

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Message-ID: <87jz2h0xyv.ffs@tglx>
Date: Tue, 02 Sep 2025 15:45:44 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>, LKML
 <linux-kernel@...r.kernel.org>
Cc: Jens Axboe <axboe@...nel.dk>, Peter Zijlstra <peterz@...radead.org>,
 "Paul E. McKenney" <paulmck@...nel.org>, Boqun Feng
 <boqun.feng@...il.com>, Paolo Bonzini <pbonzini@...hat.com>, Sean
 Christopherson <seanjc@...gle.com>, Wei Liu <wei.liu@...nel.org>, Dexuan
 Cui <decui@...rosoft.com>, x86@...nel.org, Arnd Bergmann <arnd@...db.de>,
 Heiko Carstens <hca@...ux.ibm.com>, Christian Borntraeger
 <borntraeger@...ux.ibm.com>, Sven Schnelle <svens@...ux.ibm.com>, Huacai
 Chen <chenhuacai@...nel.org>, Paul Walmsley <paul.walmsley@...ive.com>,
 Palmer Dabbelt <palmer@...belt.com>
Subject: Re: [patch V2 09/37] rseq: Introduce struct rseq_event

On Mon, Aug 25 2025 at 14:11, Mathieu Desnoyers wrote:
>> + * @sched_switch:	True if the task was scheduled out
>> + * @has_rseq:		True if the task has a rseq pointer installed
>> + */
>> +struct rseq_event {
>> +	union {
>> +		u32				all;
>> +		struct {
>> +			union {
>> +				u16		events;
>> +				struct {
>> +					u8	sched_switch;
>> +				};
>
> Is alpha still supported, or can we assume bytewise loads/stores ?

Alpha is on life support, but that does not mean we have to cater for it
in new features.

> Are those events meant to each consume 1 byte (which limits us to 2
> events for a 2-byte "events"/4-byte "all"), or is the plan to update
> them with bitwise or/~ and ?

No. Bitwise or/and is creating horrible ASM code and needs serialization
in the worst case.

There is a no need for tons of events. See changes further down the
series.

Thanks,

        tglx