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Wed, 25 Mar 2026 14:33:24 +0000 Date: Wed, 25 Mar 2026 14:33:23 +0000 Message-ID: <86h5q43rnw.wl-maz@kernel.org> From: Marc Zyngier To: Mark Rutland Cc: Sebastian Ene , kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, android-kvm@google.com, catalin.marinas@arm.com, joey.gouly@arm.com, oupton@kernel.org, suzuki.poulose@arm.com, tabba@google.com, vdonnefort@google.com, will@kernel.org, yuzenghui@huawei.com Subject: Re: [PATCH v2] KVM: arm64: Prevent the host from using an smc with imm16 != 0 In-Reply-To: References: <20260325113138.4171430-1-sebastianene@google.com> User-Agent: Wanderlust/2.15.9 (Almost Unreal) SEMI-EPG/1.14.7 (Harue) FLIM-LB/1.14.9 (=?UTF-8?B?R29qxY0=?=) APEL-LB/10.8 EasyPG/1.0.0 Emacs/30.1 (aarch64-unknown-linux-gnu) MULE/6.0 (HANACHIRUSATO) Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-SA-Exim-Connect-IP: 185.219.108.64 X-SA-Exim-Rcpt-To: mark.rutland@arm.com, sebastianene@google.com, kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, android-kvm@google.com, catalin.marinas@arm.com, joey.gouly@arm.com, oupton@kernel.org, suzuki.poulose@arm.com, tabba@google.com, vdonnefort@google.com, will@kernel.org, yuzenghui@huawei.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false On Wed, 25 Mar 2026 13:28:05 +0000, Mark Rutland wrote: >=20 > On Wed, Mar 25, 2026 at 11:31:38AM +0000, Sebastian Ene wrote: > > The ARM Service Calling Convention (SMCCC) specifies that the function > > identifier and parameters should be passed in registers, leaving the > > 16-bit immediate field of the SMC instruction un-handled. >=20 > That's not quite right; the SMCCC spec says callers must use immediate > 0. >=20 > See https://developer.arm.com/documentation/den0028/h/ section 2.10 > ("SME and HVC immediate value"), which says: >=20 > | =E2=80=A2 For all compliant calls, an SMC or HVC immediate value of zer= o must be > | used. > | =E2=80=A2 Nonzero immediate values in SMC instructions are reserved. > | =E2=80=A2 Nonzero immediate values in HVC instructions are designated f= or use by > | hypervisor vendors. >=20 > > Currently, our pKVM handler ignores the immediate value, which could le= ad > > to non-compliant software relying on implementation-defined behavior. > > Enforce the host kernel running under pKVM to use an immediate value > > of 0 by decoding the ISS from the ESR_EL2 and return a not supported > > error code back to the caller. >=20 > From my PoV, it'd be fine to turn a non-zero immediate into an UNDEF: I disagree. If SMC can be handled at all, then it cannot UNDEF based on the immediate -- the is no provision for that in the architecture. If it can UNDEF, then it must UNDEF always (as if SCR_EL3.SMD =3D=3D 1). Thanks, M. --=20 Without deviation from the norm, progress is not possible.