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 88747C4332F for ; Fri, 18 Nov 2022 17:05:47 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S242448AbiKRRFS (ORCPT ); Fri, 18 Nov 2022 12:05:18 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:43082 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S242224AbiKRREu (ORCPT ); Fri, 18 Nov 2022 12:04:50 -0500 Received: from out2.migadu.com (out2.migadu.com [IPv6:2001:41d0:2:aacc::]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id C806810FD2; Fri, 18 Nov 2022 09:04:48 -0800 (PST) Date: Fri, 18 Nov 2022 17:04:42 +0000 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux.dev; s=key1; t=1668791087; 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=TT9WIDG2PcC0Y0VIGysC67t4tuJ3IbUzD2E14K7Mj+U=; b=LGYC1eBuUtn8jOWDiDcXMIJ1YnhdPWLXut1ZBEcavASub+m8ULYW0AASixs5J/vq/3BhV1 mVS8burSTjm3iAAFWmAcG5ZCDTMyuAxBv3B0ihS0N/cgR1xxf7HIwJlJWJnK35X8d7ijLp fU7waACJ//jWMMIY9avT7eDUsKoxS+A= X-Report-Abuse: Please report any abuse attempt to abuse@migadu.com and include these headers. From: Oliver Upton To: Will Deacon Cc: Marc Zyngier , James Morse , Alexandru Elisei , Suzuki K Poulose , Catalin Marinas , Paolo Bonzini , Raghavendra Rao Ananta , linux-arm-kernel@lists.infradead.org, kvmarm@lists.linux.dev, kvmarm@lists.cs.columbia.edu, linux-kernel@vger.kernel.org, kvm@vger.kernel.org Subject: Re: [RFC PATCH 2/3] KVM: arm64: Allow userspace to trap SMCCC sub-ranges Message-ID: References: <20221110015327.3389351-1-oliver.upton@linux.dev> <20221110015327.3389351-3-oliver.upton@linux.dev> <20221118145637.GC4624@willie-the-truck> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20221118145637.GC4624@willie-the-truck> X-Migadu-Flow: FLOW_OUT Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Nov 18, 2022 at 02:56:38PM +0000, Will Deacon wrote: > On Thu, Nov 10, 2022 at 01:53:26AM +0000, Oliver Upton wrote: > > As the SMCCC (and related specifications) march towards an > > 'everything and the kitchen sink' interface for interacting with a > > system, it is less likely that KVM will implement every supported > > feature. > > > > Add a capability that allows userspace to trap hypercall ranges, > > allowing the VMM to mix-and-match between calls handled in userspace vs. > > KVM. > > > > Signed-off-by: Oliver Upton > > --- > > arch/arm64/include/asm/kvm_host.h | 5 ++++ > > arch/arm64/include/uapi/asm/kvm.h | 15 ++++++++++ > > arch/arm64/kvm/arm.c | 10 +++++++ > > arch/arm64/kvm/hypercalls.c | 48 +++++++++++++++++++++++++++++++ > > include/uapi/linux/kvm.h | 1 + > > 5 files changed, 79 insertions(+) > > [...] > > > diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c > > index 6f0b56e7f8c7..6e8a222fc295 100644 > > --- a/arch/arm64/kvm/arm.c > > +++ b/arch/arm64/kvm/arm.c > > @@ -100,6 +100,13 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm, > > r = 0; > > set_bit(KVM_ARCH_FLAG_SYSTEM_SUSPEND_ENABLED, &kvm->arch.flags); > > break; > > + case KVM_CAP_ARM_USER_HYPERCALLS: > > + if (cap->args[0] & ~KVM_ARM_USER_HYPERCALL_FLAGS) > > + return -EINVAL; > > Why not use KVM_CAP_EXIT_HYPERCALL for this? Err... I hilariously hijacked its UAPI for the exit but added a new cap for it :) I think the direction going forward will be to provide userspace with a range-based filter such that (to a degree) we can arbitrarily forward hypercalls to userspace, allowing for a mix-and-match approach. > At some point during pKVM > development, we used that to notify the VMM about memory being shared > back from the guest but we eventually dropped it as the notification to > userspace ended up not being needed: > > https://android-kvm.googlesource.com/linux/+/dbd2861832dfc4c8a3103214b3c212ee7ace1c44%5E%21/ > https://android-kvm.googlesource.com/linux/+/2a3afc6da99c0e0cb62be1687153ee572903aa80%5E%21/ > > I'm not saying that what we did was necessarily better, but it seems a bit > simpler and I figured it might be useful to point you to it. Yeah, this is certainly a lot cleaner than what I've proposed here. And frankly, for my immediate interest (forwarding vendor hypercalls to userspace), this would fit the bill. OTOH, I was hoping that something a bit more flexible could move the onus of implementing every darn spec onto userspace (where possible). I know you said pKVM has no need for userspace notifications at this moment, but could user hypercalls be useful again going forward? -- Thanks, Oliver