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 bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 5304EC982FA for ; Tue, 22 Sep 2026 14:22:46 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Content-Type:MIME-Version: Message-ID:Date:References:In-Reply-To:Subject:Cc:To:From:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=L3oO91V4IF4G60vcONMUAfnhjyKqf9fK8HxPqZ7yDx8=; b=Zkia8CJilm3ZcPfOa523HA4wie J+MCU9yVRC/h90K1I2/3fj2i8r49HZ1Vl5trj2tOCO4Lr5qZBVX9/GDwn8OeGbwHNrWg8ptzo+pbp KMYYcJYTWJFe2VeAhVj7YXPPt8F2Qb4tDsdWb//iBNJGvEbqjJDzqIHwt6AM0X5TRD2+KPFKEXplc XowQlDIEULNf0AYUNtkMu93/EBOGqIZTE4rZT0dyEI8XeT+3T+Cx2tg1fwQ8yNWDm4p2X7fW3hhfn uGBVW8ko/g9GW+qMXAA05iHBJ2TO/KnrWy+oYVWe6Uu95dDlVKi1GPdukTJSlZONgNnm2FEbWjRgj PdpiZfYg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.99.1 #2 (Red Hat Linux)) id 1x91OI-00000005fTX-3qs6; Tue, 22 Sep 2026 14:22:38 +0000 Received: from sea.source.kernel.org ([2600:3c0a:e001:78e:0:1991:8:25]) by bombadil.infradead.org with esmtps (Exim 4.99.1 #2 (Red Hat Linux)) id 1x91OH-00000005fTB-3LON for linux-arm-kernel@lists.infradead.org; Tue, 22 Sep 2026 14:22:37 +0000 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id 74D7643CD3; Tue, 22 Sep 2026 14:22:37 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 145C31F000FF; Tue, 22 Sep 2026 14:22:28 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1790086957; bh=L3oO91V4IF4G60vcONMUAfnhjyKqf9fK8HxPqZ7yDx8=; h=From:To:Cc:Subject:In-Reply-To:References:Date; b=oKHS5DRaI08HjzcvWtYe1RxTCulSKE9K7GAetzK1gAmlKpTZtwVOLsZGx+vxwsIoF 4Fc0Mpej6Jcl6qB59u6qmZuL/+YTub0JsQVybmm6RTyI7v0Jc4UxNynINCYcqAYyW0 NYoQhMY5BLRmt+GhwBGFKduN/BOXph8W1gyKSjcp7z+JdYuGyjY7WwsNHrETDFwZ0a nyH5Hqi31sadWwRn4whOeg/UOMWCcDyVHsD4okuN88AvFXXac7qasNQ1QrBwPhQqN7 Fl2HjDnnomdmMKFbIv9ftdOEOq/4D5bQZpbVWAvFeve0a6jmcSu42XdS8Mh5BuB2Qj WY0Nem/ttVZzg== X-Mailer: emacs 31.1 (via feedmail 11-beta-1 I) From: Aneesh Kumar K.V To: "Lorenzo Stoakes (ARM)" Cc: Catalin Marinas , Will Deacon , Marc Zyngier , Oliver Upton , Fuad Tabba , Joey Gouly , Steffen Eiden , Suzuki K Poulose , Zenghui Yu , Paolo Bonzini , Jonathan Corbet , linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, kvmarm@lists.linux.dev, kvm@vger.kernel.org, linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org, Jack Thomson , Jack Thomson , Alexandru Elisei , Vincent Donnefort , Sean Christopherson , Claudio Imbrenda , Leo Soares Passos Subject: Re: [PATCH v2 00/13] KVM: arm64: Add KVM_PRE_FAULT_MEMORY support In-Reply-To: References: <20260914-kvm-arm-prefault-v2-0-26fb47f74b73@kernel.org> Date: Tue, 22 Sep 2026 19:52:25 +0530 Message-ID: MIME-Version: 1.0 Content-Type: text/plain 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 "Lorenzo Stoakes (ARM)" writes: > On Tue, Sep 22, 2026 at 06:37:04PM +0530, Aneesh Kumar K.V wrote: >> "Lorenzo Stoakes (ARM)" writes: >> >> > On Tue, Sep 22, 2026 at 03:28:33PM +0530, Aneesh Kumar K.V wrote: >> >> > This is because pKVM instantiates vCPUs upon run, >> >> > >> >> >> >> Can pKVM instantiate the hyp vCPU during pre-faulting ? >> > >> > It would be an unusual and unexpected thing to do - suddenly a pre-fault >> > operation is initialising a vCPU explicitly for pKVM. >> > >> > A caller is not going to reasonably expect this and might treat a failure to >> > pre-alloc as fine to carry on whereas in fact it was a failure to initailised a >> > pKVM vCPU. >> > >> > It'd also require significant changes to how pKVM is set up, right now it's >> > hardcoded to be done unconditionally at run via kvm_arch_vcpu_run_pid_change() >> > -> pkvm_create_hyp_vcpu(), so all that would have to change and be checked and >> > tested and... that'd be really out of scope I think :) >> > >> > And pre-faulting really makes most sense BEFORE you run a VM. It doesn't make so >> > much sense mid-run. >> > >> > But more fundamentally, the stage 2 page tables, as I understand it, are owned >> > by pKVM and so aren't really available to be pre-faulted. >> > >> > Maybe unprotected-under-pKVM VMs but then it's questionable as to how useful >> > that would be given that it would be confusing to users vs. how it works for >> > other VMs. >> > >> > So in general, no I don't think it's a good idea. >> > >> > And even if we wanted to pursue some version of this, it's _definitely_ out >> > of scope for the initial pre-faulting bring-up series. >> > >> >> >> >> >> >> > but pre-faulting is typically performed before a vCPU is run. It would be confusing and >> >> > inconsistent to error out on non-running vCPUs but to pre-fault running >> >> > ones. >> >> >> >> >> >> I use KVM pre-faulting when transitioning pages from shared to private >> > >> > You mean you'd prefer to use? Or you are using it on another arch? >> > >> >> with CoCo guest. This ensures that a trusted device can DMA to private >> >> memory before the guest accesses it. >> > >> > Hm what do you mean by private memory? >> > >> > I see: >> > >> > #ifndef CONFIG_KVM_GENERIC_MEMORY_ATTRIBUTES >> > static inline bool kvm_arch_has_private_mem(struct kvm *kvm) >> > { >> > return false; >> > } >> > #endif >> > >> > And only x86 selects KVM_GENERIC_MEMORY_ATTRIBUTES? >> > >> > Do you mean something else? >> > >> >> I am using this with ARM CCA-DA, based on the patch series from Jack Thomson . >> >> https://gitlab.arm.com/linux-arm/kvmtool-cca/-/commit/80e7aad61c5639de2f0cb4a5525dad0c96156428 >> >> We do this while the VM is running. > > Right, that's a non-mainline kernel I guess? Which presumably implements private memory. > > Jack himself experienced a panic with his pKVM code, so the code you're using is > not upstreamable, unfortunately. And he'd already shelved pKVM support AFAICT. > > And reviewers pointed out actually implementing the pKVM stuff properly would be > quite involved, even if you wanted to do that (hence follow-up). > > Also you end up stuck with the same problems as I mentioned above - you can't > sanely bring the vCPU pre-run, so now you have extremely weird behaviour - only > pre-faults if vCPU initialised, running, and unprotected pKVM. > IIUC, pkvm_pgtable_stage2_map() only uses the hyp vCPU's pKVM memcache (&vcpu->vcpu.arch.pkvm_memcache). I agree that this does not need to be addressed in this series. However, there is also a desire to keep the CCA and pKVM code paths similar by using helpers such as kvm_vm_is_protected(). Since an RMM can create a stage-2 mapping without a REC (vCPU), rejecting pre-faulting for all protected VM configurations may be incorrect. I can handle this in my series though. I also want to point out that there are use cases for pre-faulting while the VM is running. > > Protected pKVM support is a whole other layer of complexity and it's not obvious > that you're really achieving what pre-fault is supposed to. > > In any case Oliver literally just asked me to _simplify_ weird edge cases for > this series :) so I am not sure something like that is going to be accepted. > > Are you sure you're actually running in pKVM mode btw? CCA doesn't AFAICT? In > which case this series _should_ work fine for you. > It is not pKVM; it runs in Realm mode. > > Anyway, if we really do need to add something for pKVM it needs to be a follow > up. Let's get the basics working first :) > sure. > (Note that kvmtool will need to be updated to retry pre-fault on -EAGAIN, -EINTR > as this series can, albeit unlikely, return -EAGAIN.) > I will check this when I rebase my kernel onto this series. -aneesh