From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6E141CD37AD for ; Fri, 15 Sep 2023 21:54:50 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236560AbjIOVyX (ORCPT ); Fri, 15 Sep 2023 17:54:23 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:49240 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236926AbjIOVyT (ORCPT ); Fri, 15 Sep 2023 17:54:19 -0400 Received: from out-222.mta0.migadu.com (out-222.mta0.migadu.com [91.218.175.222]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 9058D139 for ; Fri, 15 Sep 2023 14:54:12 -0700 (PDT) Date: Fri, 15 Sep 2023 21:54:05 +0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1694814850; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=IRK5mFLmf/ekVrKw/W6wumPqbLTXGlC7g336g390ZFg=; b=HM4B49eRkbN+AztPqOtcIBPQC13taubkjpx98EkeUP/8Y+6/ptecJDrBvfnA/jQphRy9pj 2LFFJMqKbLJ6s3StvU6XlyXPsr0+lYYUqilCc/R79wnSGIeoyFR+griit9Y7i+qyOq48ee oT5hUVuAEb96mZXronFcO5nwBXzvVHI= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Oliver Upton To: Raghavendra Rao Ananta Cc: Marc Zyngier , Alexandru Elisei , James Morse , Suzuki K Poulose , Paolo Bonzini , Zenghui Yu , Shaoqin Huang , Jing Zhang , Reiji Watanabe , Colton Lewis , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, linux-kernel@vger.kernel.org, kvm@vger.kernel.org Subject: Re: [PATCH v5 08/12] KVM: arm64: PMU: Allow userspace to limit PMCR_EL0.N for the guest Message-ID: References: <20230817003029.3073210-1-rananta@google.com> <20230817003029.3073210-9-rananta@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-Migadu-Flow: FLOW_OUT Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On Fri, Sep 15, 2023 at 08:53:16PM +0000, Oliver Upton wrote: > Hi Raghu, > > On Thu, Aug 17, 2023 at 12:30:25AM +0000, Raghavendra Rao Ananta wrote: > > From: Reiji Watanabe > > > > KVM does not yet support userspace modifying PMCR_EL0.N (With > > the previous patch, KVM ignores what is written by upserspace). > > typo: userspace > > > diff --git a/arch/arm64/kvm/pmu-emul.c b/arch/arm64/kvm/pmu-emul.c > > index ce7de6bbdc967..39ad56a71ad20 100644 > > --- a/arch/arm64/kvm/pmu-emul.c > > +++ b/arch/arm64/kvm/pmu-emul.c > > @@ -896,6 +896,7 @@ int kvm_arm_set_vm_pmu(struct kvm *kvm, struct arm_pmu *arm_pmu) > > * while the latter does not. > > */ > > kvm->arch.pmcr_n = arm_pmu->num_events - 1; > > + kvm->arch.pmcr_n_limit = arm_pmu->num_events - 1; > > Can't we just get at this through the arm_pmu instance rather than > copying it into kvm_arch? > > > return 0; > > } > > diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c > > index 2075901356c5b..c01d62afa7db4 100644 > > --- a/arch/arm64/kvm/sys_regs.c > > +++ b/arch/arm64/kvm/sys_regs.c > > @@ -1086,6 +1086,51 @@ static int get_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r, > > return 0; > > } > > > > +static int set_pmcr(struct kvm_vcpu *vcpu, const struct sys_reg_desc *r, > > + u64 val) > > +{ > > + struct kvm *kvm = vcpu->kvm; > > + u64 new_n, mutable_mask; > > + int ret = 0; > > + > > + new_n = FIELD_GET(ARMV8_PMU_PMCR_N, val); > > + > > + mutex_lock(&kvm->arch.config_lock); > > + if (unlikely(new_n != kvm->arch.pmcr_n)) { > > + /* > > + * The vCPU can't have more counters than the PMU > > + * hardware implements. > > + */ > > + if (new_n <= kvm->arch.pmcr_n_limit) > > + kvm->arch.pmcr_n = new_n; > > + else > > + ret = -EINVAL; > > + } > > Hmm, I'm not so sure about returning an error here. ABI has it that > userspace can write any value to PMCR_EL0 successfully. Can we just > ignore writes that attempt to set PMCR_EL0.N to something higher than > supported by hardware? Our general stance should be that system register > fields responsible for feature identification are immutable after the VM > has started. I hacked up my reply and dropped some context; this doesn't read right. Shaoqin made the point about preventing changes to PMCR_EL0.N after the VM has started and I firmly agree. The behavior should be: - Writes to PMCR always succeed - PMCR_EL0.N values greater than what's supported by hardware are ignored - Changes to N after the VM has started are ignored. -- Thanks, Oliver