From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from foss.arm.com (foss.arm.com [217.140.110.172]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 2ACBA3C8733; Thu, 10 Sep 2026 10:27:59 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=217.140.110.172 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789036080; cv=none; b=Qpp/LNKwPAGHr1j4lBvWtIPNXOIX/AjBdHZfGonFbtZNuqr1MFhW+ZCXQ/jbarXZr5ginIXYe1FjlHroZn2d7ReMTJOK9woAaoRExtSqNjJekyMJhKhRevx2z5QopXnMCMtnnwvhuk+pdNmMTcydThhtUP1O9/mjlWQnd3X2aEI= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1789036080; c=relaxed/simple; bh=aWQt/+Tvo/ggfRTd82k5+d6xlXw8sI7UgTyynHOeLvI=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=HsjgUsh+IvACOco7ge83wrpyPh1AmVPbCHZjMHOn/LJFfDSZNi80Iy8O7hggkuGynibbGIGKBkIXtxaUs7Fh1HnRMDW18B2jjLIlzccSRK4H9n8bXXJB/CMmy90UGnxGt+/ZCbAMcDjQ3eQbiVx3HHqyLhIFB6xpsdu38HSD/Rc= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com; spf=pass smtp.mailfrom=arm.com; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b=KYlafta4; arc=none smtp.client-ip=217.140.110.172 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=arm.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=arm.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=arm.com header.i=@arm.com header.b="KYlafta4" Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id C4E8D143D; Thu, 10 Sep 2026 03:27:54 -0700 (PDT) Received: from [10.0.129.245] (unknown [10.0.129.245]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 7E7D23F7B4; Thu, 10 Sep 2026 03:27:55 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=simple/simple; d=arm.com; s=foss; t=1789036078; bh=aWQt/+Tvo/ggfRTd82k5+d6xlXw8sI7UgTyynHOeLvI=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=KYlafta4EkjMZWOAfvdk24/OlQIy6uIrL0Nvg71duaejrS5Ng65DY7mkcYqZLh0um 5Im0f/4WGHfB13DFvJ55+kmxVIWDjNT3YvQ4WS4QtFD4gpq3nRSLPdfuh5Aai2eUDj Sizaon0gsCgqril8nrlevqCxDdy7//RcewiBGgl4= Message-ID: Date: Thu, 10 Sep 2026 11:27:54 +0100 Precedence: bulk X-Mailing-List: kvmarm@lists.linux.dev List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v17 10/20] KVM: arm64: coco: Disable Steal time accounting for coco guests Content-Language: en-GB To: Fuad Tabba Cc: kvm@vger.kernel.org, kvmarm@lists.linux.dev, maz@kernel.org, will@kernel.org, catalin.marinas@arm.com, linux-kernel@vger.kernel.org, linux-arm-kernel@lists.infradead.org, steven.price@arm.com, aneesh.kumar@kernel.org, oupton@kernel.org, gshan@redhat.com, joey.gouly@arm.com, yuzenghui@huawei.com, linux-coco@lists.linux.dev, gankulkarni@os.amperecomputing.com, sdonthineni@nvidia.com, alpergun@google.com, fj0570is@fujitsu.com, WeiLin.Chang@arm.com, lpieralisi@kernel.org, enju.kohei@fujitsu.com References: <20260908162223.1683432-1-suzuki.poulose@arm.com> <20260908162223.1683432-11-suzuki.poulose@arm.com> <6152fc41-65f0-460f-90b4-1454c11bcba6@arm.com> From: Suzuki K Poulose In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Hi Fuad On 09/09/2026 13:23, Fuad Tabba wrote: > Hi Suzuki, > > On Wed, 9 Sept 2026 at 12:52, Suzuki K Poulose wrote: > ... > >>> I think it would be better to leave KVM_CAP_STEAL_TIME reported and >>> instead reject the configuration for these VMs, returning -EPERM from >>> the KVM_ARM_VCPU_PVTIME_CTRL attr. A capability query should report >>> what the kernel supports, not whether a given VM may use it; gating it >>> per-VM here is also inconsistent, since the !kvm clause leaves the >>> system-fd cap at 1 while the VM-fd cap reads 0. And -EPERM says that >> >> >> But isn't that we want from a VMM perspective ? i.e., enable PVTIME if >> it is supported for the given VM (type). Rather than PVTIME is supported >> by the KVM, so I can go enable it by default. But, we endup getting a >> failure while we enable it. I encountered this with kvmtool, where >> CAP_STEAL_TIME is reported true and even the HAS_DEVICE_ATTR goes >> through fine, but SET_DEVICE_ATTR seemed to fail. >> >> Isn't it better to allow the VM to decide if the capability is supported >> for the given VM type, like we are doing for other capabilities ? > > You're right, per-VM capability reporting. My wires got crossed from > the pKVM series and the kvmtool fixes. However, I'd still return > -EPERM rather than -ENXIO from the attribute. The capability exists, Just to make sure I understand the picture: For coco VMs: - check extension on coco KVM, KVM_CAP_STEAL_TIME => 0 And a follow up DEVICE_HAS_ATTR => -EPERM ? But we tell the VMM, look you don't have the capabilit on this VM. But if you try to set the attribute isn't it more appropriate to say: -ENXIO - No you don't have that feature on this VM (-ENXIO) rather than -EPERM - No, I won't let you do that. Remember we are doing only in the "VM" context. It is a minor nit but I wanted to make sure we understand what we are thinking. Cheers Suzuki > it's just not permitted for a confidential VM, and -EPERM says exactly > that, whereas -ENXIO reads as "no such feature". > > Cheers, > /fuad > >> >>> steal-time exists but isn't permitted for a confidential VM, whereas >>> -ENXIO reads as "no such feature". >> >> Cheers >> Suzuki >> >> >>> >>> Cheers, >>> /fuad >>> >>>> >>>> int kvm_arm_pvtime_set_attr(struct kvm_vcpu *vcpu, >>>> @@ -81,7 +81,7 @@ int kvm_arm_pvtime_set_attr(struct kvm_vcpu *vcpu, >>>> int ret = 0; >>>> int idx; >>>> >>>> - if (!kvm_arm_pvtime_supported() || >>>> + if (!kvm_arm_pvtime_supported(kvm) || >>>> attr->attr != KVM_ARM_VCPU_PVTIME_IPA) >>>> return -ENXIO; >>>> >>>> @@ -110,7 +110,7 @@ int kvm_arm_pvtime_get_attr(struct kvm_vcpu *vcpu, >>>> u64 __user *user = (u64 __user *)attr->addr; >>>> u64 ipa; >>>> >>>> - if (!kvm_arm_pvtime_supported() || >>>> + if (!kvm_arm_pvtime_supported(vcpu->kvm) || >>>> attr->attr != KVM_ARM_VCPU_PVTIME_IPA) >>>> return -ENXIO; >>>> >>>> @@ -126,7 +126,7 @@ int kvm_arm_pvtime_has_attr(struct kvm_vcpu *vcpu, >>>> { >>>> switch (attr->attr) { >>>> case KVM_ARM_VCPU_PVTIME_IPA: >>>> - if (kvm_arm_pvtime_supported()) >>>> + if (kvm_arm_pvtime_supported(vcpu->kvm)) >>>> return 0; >>>> } >>>> return -ENXIO; >>>> -- >>>> 2.43.0 >>>> >>