Re: Auto-enabling V unit and/or use of elf attributes (was Re: Adding V-ext regs to signal context w/o expanding kernel struct sigcontext to avoid glibc ABI break)

From: Jeff Law <jlaw@ventanamicro.com>
To: Richard Henderson <richard.henderson@linaro.org>,
	Vineet Gupta <vineetg@rivosinc.com>,
	Kito Cheng <kito.cheng@sifive.com>
Cc: "Philipp Tomsich" <philipp.tomsich@vrull.eu>,
	"Andy Chiu" <andy.chiu@sifive.com>,
	"Vincent Chen" <vincent.chen@sifive.com>,
	"Florian Weimer" <fweimer@redhat.com>,
	"Rich Felker" <dalias@libc.org>,
	"Andrew Waterman" <andrew@sifive.com>,
	"Palmer Dabbelt" <palmer@rivosinc.com>,
	"Christoph Müllner" <christoph.muellner@vrull.eu>,
	davidlt@rivosinc.com, "Arnd Bergmann" <arnd@arndb.de>,
	"Björn Töpel" <bjorn@kernel.org>,
	"Szabolcs Nagy" <szabolcs.nagy@arm.com>,
	"Greentime Hu" <greentime.hu@sifive.com>,
	"Aaron Durbin" <adurbin@rivosinc.com>,
	"Andrew de los Reyes" <adlr@rivosinc.com>,
	linux-riscv <linux-riscv@lists.infradead.org>,
	"GNU C Library" <libc-alpha@sourceware.org>
Subject: Re: Auto-enabling V unit and/or use of elf attributes (was Re: Adding V-ext regs to signal context w/o expanding kernel struct sigcontext to avoid glibc ABI break)
Date: Tue, 10 Jan 2023 22:07:39 -0700	[thread overview]
Message-ID: <6d13e63f-69b3-6e48-b811-bbfcf3ffb3af@ventanamicro.com> (raw)
In-Reply-To: <8be4d673-f435-429e-9a61-bb49e7820529@linaro.org>


On 1/10/23 21:57, Richard Henderson wrote:
> On 1/10/23 20:28, Jeff Law wrote:
>>
>>
>> On 1/10/23 18:22, Richard Henderson wrote:
>>> On 1/10/23 10:07, Vineet Gupta wrote:
>>>> Yes bulk of glibc might not have vector code, but those V ifunc 
>>>> routines do and IMO this information needs to be recorded somewhere 
>>>> in the elf. Case in point being the current issue with how to enable 
>>>> V unit. Community wants a per-process enable, using an explicit 
>>>> prctl from userspace (since RV doesn't have fault-on-first use 
>>>> hardware mechanism unlike some of the other arches). But how does 
>>>> the glibc loader know to invoke prctl. We can't just rely on user 
>>>> env GLIBC_TUNABLE etc since that might not be accurate. It needs 
>>>> somethign concrete which IMO can come from elf attributes. If not, 
>>>> do you have suggestions on how to solve this issue ?
>>>
>>> Why not just fault on first use to enable?  That's vastly less 
>>> complicated than trying to plumb anything through elf resulting in a 
>>> prctl.
>> Well, the answer is in Vineet's paragraph -- the hardware apparently 
>> doesn't have fault-on-first-use which is mighty unfortunate.
> 
> Nonsense -- sstatus.vs stores {off, initial, clean, dirty} state, just 
> like fpu.
> Now treat the vector unit just like fpu lazy migration.
Then let's do something sensible.    Manually enabling via prctl seems 
silly if we have fault on first use.

jeff

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