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Thu, 09 Oct 2025 14:10:11 +0000 Date: Thu, 09 Oct 2025 15:10:07 +0100 Message-ID: <86sefsxj28.wl-maz@kernel.org> From: Marc Zyngier To: Mark Rutland Cc: linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, Joey Gouly , Suzuki K Poulose , Oliver Upton , Zenghui Yu , Will Deacon , Catalin Marinas , Jan Kotas Subject: Re: [PATCH] arm64: Revamp HCR_EL2.E2H RES1 detection In-Reply-To: References: <20251009121239.29370-1-maz@kernel.org> 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: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 (generated by SEMI-EPG 1.14.7 - "Harue") Content-Type: text/plain; charset=US-ASCII X-SA-Exim-Connect-IP: 185.219.108.64 X-SA-Exim-Rcpt-To: mark.rutland@arm.com, linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, joey.gouly@arm.com, suzuki.poulose@arm.com, oliver.upton@linux.dev, yuzenghui@huawei.com, will@kernel.org, catalin.marinas@arm.com, jank@cadence.com X-SA-Exim-Mail-From: maz@kernel.org X-SA-Exim-Scanned: No (on disco-boy.misterjones.org); SAEximRunCond expanded to false On Thu, 09 Oct 2025 14:00:22 +0100, Mark Rutland wrote: > > On Thu, Oct 09, 2025 at 01:12:39PM +0100, Marc Zyngier wrote: > > We currently have two ways to identify CPUs that only implement FEAT_VHE > > and not FEAT_E2H0: > > > > - either they advertise it via ID_AA64MMFR4_EL1.E2H0, > > - or the HCR_EL2.E2H bit is RAO/WI > > > > However, there is a third category of "cpus" that fall between these > > two cases: on CPUs that do not implement FEAT_FGT, it is IMPDEF whether > > an access to ID_AA64MMFR4_EL1 can trap to EL2 when the register value > > is zero. > > > > A consequence of this is that on systems such as Neoverse V2, a NV > > guest cannot reliably detect that it is in a VHE-only configuration > > (E2H is writable, and ID_AA64MMFR0_EL1 is 0), despite the hypervisor's > > best effort to repaint the id register. > > > > Replace the RAO/WI test by a sequence that makes use of the VHE > > register remnapping between EL1 and EL2 to detect this situation, > > and work out whether we get the VHE behaviour even after having > > set HCR_EL2.E2H to 0. > > > > This solves the NV problem, and provides a more reliable acid test > > for CPUs that do not completely follow the letter of the architecture > > while providing a RES1 behaviour for HCR_EL2.E2H. > > > > Suggested-by: Marc Rutland > > Signed-off-by: Marc Zyngier > > Link: https://lore.kernel.org/r/15A85F2B-1A0C-4FA7-9FE4-EEC2203CC09E@global.cadence.com > > --- > > arch/arm64/include/asm/el2_setup.h | 30 ++++++++++++++++++++++++------ > > 1 file changed, 24 insertions(+), 6 deletions(-) > > > > diff --git a/arch/arm64/include/asm/el2_setup.h b/arch/arm64/include/asm/el2_setup.h > > index 46033027510cc..b7640e2c20503 100644 > > --- a/arch/arm64/include/asm/el2_setup.h > > +++ b/arch/arm64/include/asm/el2_setup.h > > @@ -24,22 +24,40 @@ > > * ID_AA64MMFR4_EL1.E2H0 < 0. On such CPUs HCR_EL2.E2H is RES1, but it > > * can reset into an UNKNOWN state and might not read as 1 until it has > > * been initialized explicitly. > > - * > > - * Fruity CPUs seem to have HCR_EL2.E2H set to RAO/WI, but > > - * don't advertise it (they predate this relaxation). > > - * > > * Initalize HCR_EL2.E2H so that later code can rely upon HCR_EL2.E2H > > * indicating whether the CPU is running in E2H mode. > > */ > > mrs_s x1, SYS_ID_AA64MMFR4_EL1 > > sbfx x1, x1, #ID_AA64MMFR4_EL1_E2H0_SHIFT, #ID_AA64MMFR4_EL1_E2H0_WIDTH > > cmp x1, #0 > > - b.ge .LnVHE_\@ > > + b.lt .LnE2H0_\@ > > > > > + /* > > + * Fruity CPUs seem to have HCR_EL2.E2H set to RAO/WI, but don't > > + * advertise it (they predate this relaxation). Check for an > > + * essential VHE property (system register remapping) to decide > > + * whether we're effectively VHE-only or not. > > + * > > + * This is also useful for for NV guests on CPUs that can't trap > > + * ID_AA64MMFR4_EL1 as they don't have FEAT_FGT. > > + */ > > Would you be happy to elaborate this comment to: > > /* > * Unfortunately, HCR_EL2.E2H can be RES1 even if not advertised > * as such via ID_AA64MMFR4_EL1.E2H0: > * > * - Fruity CPUs predate the !FEAT_E2H0 relaxation, and seem to > * have HCR_EL2.E2H implemented as RAO/WI. > * > * - On CPUs that lack FEAT_FGT, a hypervisor can't trap guest > * reads of ID_AA64MMFR4_EL1 to advertise !FEAT_E2H0. NV > * guests on these hosts can write to HCR_EL2.E2H without > * trapping to the hypervisor, but these writes have no > * functional effect. > * > * Handle both cases by checking for an essential VHE property > * (system register remapping) to decide whether we're > * effectively VHE-only or not. > */ Yup. Applied. > > Other than that, this all looks good to me: > > Acked-by: Mark Rutland Thanks! M. -- Without deviation from the norm, progress is not possible.