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From: Oliver Upton To: Marc Zyngier Cc: kvmarm@lists.linux.dev, linux-arm-kernel@lists.infradead.org, kvm@vger.kernel.org, Joey Gouly , Suzuki K Poulose , Zenghui Yu , Bjorn Andersson , Christoffer Dall , Ganapatrao Kulkarni , Chase Conklin , Eric Auger Subject: Re: [PATCH v2 01/12] KVM: arm64: nv: Add handling of EL2-specific timer registers Message-ID: References: <20241217142321.763801-1-maz@kernel.org> <20241217142321.763801-2-maz@kernel.org> <874j2xs6hz.wl-maz@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <874j2xs6hz.wl-maz@kernel.org> X-Migadu-Flow: FLOW_OUT X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20241221_135851_791478_02CEF42B X-CRM114-Status: GOOD ( 23.58 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org On Sat, Dec 21, 2024 at 09:57:44AM +0000, Marc Zyngier wrote: > On Sat, 21 Dec 2024 01:38:28 +0000, > Oliver Upton wrote: > > > > On Tue, Dec 17, 2024 at 02:23:09PM +0000, Marc Zyngier wrote: > > > @@ -3879,9 +4020,11 @@ static const struct sys_reg_desc cp15_64_regs[] = { > > > { SYS_DESC(SYS_AARCH32_CNTPCT), access_arch_timer }, > > > { Op1( 1), CRn( 0), CRm( 2), Op2( 0), access_vm_reg, NULL, TTBR1_EL1 }, > > > { Op1( 1), CRn( 0), CRm(12), Op2( 0), access_gic_sgi }, /* ICC_ASGI1R */ > > > + { SYS_DESC(SYS_AARCH32_CNTVCT), access_arch_timer }, > > > { Op1( 2), CRn( 0), CRm(12), Op2( 0), access_gic_sgi }, /* ICC_SGI0R */ > > > { SYS_DESC(SYS_AARCH32_CNTP_CVAL), access_arch_timer }, > > > { SYS_DESC(SYS_AARCH32_CNTPCTSS), access_arch_timer }, > > > + { SYS_DESC(SYS_AARCH32_CNTVCTSS), access_arch_timer }, > > > }; > > > > Huh. You know, I had always thought we hid 32-bit EL0 from nested > > guests, but I now realize that isn't the case. Of course, we don't have > > the necessary trap reflection for exits that came out of a 32-bit EL0, > > nor should we bother. > > > > Of the 4 NV2 implementations I'm aware of (Neoverse-V1, Neoverse-V2, > > AmpereOne, M2) only Neoverse-V1 supports 32-bit userspace. And even > > then, a lot of deployments of V1 have a broken NV2 implementation. > > > > What do you think about advertising a 64-bit only EL0 for nested VMs? > > I'm completely OK with that. > > Actually, we already nuke the guest if exiting from 32bit context, no > matter the EL (vcpu_mode_is_bad_32bit() is where this happens). But > we're missing the ID_AA64PFR0_EL1.EL0 sanitising, which is a bug. I'll > send a patch shortly. > > Now, for this particular patch, I still think we should gracefully > handle access to the EL1 timer from a 32bit capable, non-NV guest. > Just in case we end-up with a CPU with a broken CNTVOFF_EL2 *and* > 32bit capability. > > In the end, it doesn't cost us much to support this case, and it helps > that we can verify that we handle all registers without exception. > > Thoughts? Absolutely. The only reason I made the comment is because the 32-bit changes had nudged me into thinking about it. Happy with the patch as is. -- Thanks, Oliver