linux-kernel - Re: [PATCH 1/6] arm64/sme: Flush foreign register state in do_sme

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Message-ID: <9365be76-8da6-47ce-b88e-dfa244b9e5b7@sirena.org.uk>
Date: Tue, 3 Dec 2024 16:00:45 +0000
From: Mark Brown <broonie@...nel.org>
To: Dave Martin <Dave.Martin@....com>
Cc: Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, Mark Rutland <mark.rutland@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	stable@...r.kernel.org
Subject: Re: [PATCH 1/6] arm64/sme: Flush foreign register state in
 do_sme_acc()

On Tue, Dec 03, 2024 at 03:32:22PM +0000, Dave Martin wrote:
> On Tue, Dec 03, 2024 at 12:45:53PM +0000, Mark Brown wrote:

> > @@ -1460,6 +1460,8 @@ void do_sme_acc(unsigned long esr, struct pt_regs *regs)
> >  		sme_set_vq(vq_minus_one);
> >  
> >  		fpsimd_bind_task_to_cpu();
> > +	} else {
> > +		fpsimd_flush_task_state(current);

> TIF_FOREIGN_FPSTATE is (or was) a cache of the task<->CPU binding that
> you're clobbering here.

> So, this fpsimd_flush_task_state() should have no effect unless
> TIF_FOREIGN_FPSTATE is already wrong?  I'm wondering if the apparent
> need for this means that there is an undiagnosed bug elsewhere.

> (My understanding is based on FPSIMD/SVE; I'm less familiar with the
> SME changes, so I may be missing something important here.)

It's to ensure that the last recorded CPU for the current task is
invalid so that if the state was loaded on another CPU and we switch
back to that CPU we reload the state from memory, we need to at least
trigger configuration of the SME VL.

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