linux-kernel - Re: [PATCH v2 4/6] riscv: vector: allow to force vector context save

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Message-ID: <aPVYIOz8XRZ-737r@curiosity>
Date: Mon, 20 Oct 2025 00:29:04 +0300
From: Sergey Matyukevich <geomatsi@...il.com>
To: Andy Chiu <andybnac@...il.com>
Cc: linux-riscv@...ts.infradead.org, linux-kselftest@...r.kernel.org,
	linux-kernel@...r.kernel.org, Paul Walmsley <pjw@...nel.org>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>, Alexandre Ghiti <alex@...ti.fr>,
	Oleg Nesterov <oleg@...hat.com>, Shuah Khan <shuah@...nel.org>,
	Jisheng Zhang <jszhang@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Thomas Huth <thuth@...hat.com>,
	Charlie Jenkins <charlie@...osinc.com>,
	Han Gao <rabenda.cn@...il.com>,
	Samuel Holland <samuel.holland@...ive.com>,
	Nam Cao <namcao@...utronix.de>,
	Joel Granados <joel.granados@...nel.org>,
	Clément Léger <cleger@...osinc.com>,
	Conor Dooley <conor.dooley@...rochip.com>
Subject: Re: [PATCH v2 4/6] riscv: vector: allow to force vector context save

On Wed, Oct 15, 2025 at 04:32:05PM -0500, Andy Chiu wrote:
> On Wed, Oct 15, 2025 at 3:18 PM Andy Chiu <andybnac@...il.com> wrote:
> >
> > On Tue, Oct 7, 2025 at 6:58 AM Sergey Matyukevich <geomatsi@...il.com> wrote:
> > >
> > > When ptrace updates vector CSR registers for a traced process, the
> > > changes may not be immediately visible to the next ptrace operations
> > > due to vector context switch optimizations.
> > >
> > > The function 'riscv_v_vstate_save' saves context only if mstatus.VS is
> > > 'dirty'. However mstatus.VS of the traced process context may remain
> > > 'clean' between two breakpoints, if no vector instructions were executed
> > > between those two breakpoints. In this case the vector context will not
> > > be saved at the second breakpoint. As a result, the second ptrace may
> > > read stale vector CSR values.
> >
> > IIUC, the second ptrace should not get the stale vector CSR values.
> > The second riscv_vr_get() should be reading from the context memory
> > (vstate), which is updated from the last riscv_vr_set(). The user's
> > vstate should remain the same since last riscv_vr_set(). Could you
> > explain more on how this bug is observed and why only CSRs are
> > affected but not v-regs as well?
> 
> From looking into your test, I can see that you were trying to set an
> invalid configuration to Vetor CSRs and expect vill to be reflected
> upon next read. Yes, this is not happening on the current
> implementation as it was not expecting invalid input from the user,
> which should be taken into consideration. Thanks for spotting the
> case!
> 
> According to the spec, "The use of vtype encodings with LMUL <
> SEWMIN/ELEN is reserved, implementations can set vill if they do not
> support these configurations." This mean the implementation may
> actually support this configuration. If that is the case, I think we
> should not allow this to be configured through the vector ptrace
> interface, which is designed to support 1.0 (and 0.7) specs. That
> means, we should not allow this problematic configuration to pass
> through riscv_vr_set(), reach user space, then the forced save.
> 
> I would opt for validating all CSR configurations in the first place.
> Could you also help enforce checks on other reserved bits as well?

Just to clarify, the suggestion is to drop the TIF_RISCV_V_FORCE_SAVE
entirely and use only careful validation of input parameter in riscv_vr_set,
rather than using both checks. Is that correct?

If that is correct, then I assume we can rely on the simple rule ELEN == XLEN
to validate vsew/vlmul supported combinations. Additionally, reserved vsew
values (see 3.4.1 in spec) should also be rejected.

Thanks,
Sergey