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 X-Spam-Level: X-Spam-Status: No, score=-15.9 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 57D8EC4708F for ; Tue, 1 Jun 2021 10:24:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 36907613B9 for ; Tue, 1 Jun 2021 10:24:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S233437AbhFAK01 (ORCPT ); Tue, 1 Jun 2021 06:26:27 -0400 Received: from mga07.intel.com ([134.134.136.100]:52378 "EHLO mga07.intel.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S233237AbhFAK0Z (ORCPT ); Tue, 1 Jun 2021 06:26:25 -0400 IronPort-SDR: PaLB3wxELZMPFCJ9IqARPyAWB0C17hvPNgjonhEokj+3xSTG/hWHEdbaAjvFmaivo7Z3kBkFvq UJDBmrGLTYbA== X-IronPort-AV: E=McAfee;i="6200,9189,10001"; a="267394383" X-IronPort-AV: E=Sophos;i="5.83,239,1616482800"; d="scan'208";a="267394383" Received: from orsmga001.jf.intel.com ([10.7.209.18]) by orsmga105.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 01 Jun 2021 03:24:39 -0700 IronPort-SDR: l6ts3JXUttWZArQyQcYLDP37RmNoYkQTQcxEfj3z5zQFvY/xK94BWjEiyAV81QTrBIjw++QuTv 5Fx+Grb3y1bQ== X-IronPort-AV: E=Sophos;i="5.83,239,1616482800"; d="scan'208";a="479226991" Received: from liujing-mobl.ccr.corp.intel.com (HELO [10.238.130.133]) ([10.238.130.133]) by orsmga001-auth.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 01 Jun 2021 03:24:36 -0700 Subject: Re: [PATCH RFC 4/7] kvm: x86: Add new ioctls for XSAVE extension To: Sean Christopherson Cc: pbonzini@redhat.com, kvm@vger.kernel.org, linux-kernel@vger.kernel.org, jing2.liu@intel.com References: <20210207154256.52850-1-jing2.liu@linux.intel.com> <20210207154256.52850-5-jing2.liu@linux.intel.com> <920df897-56d8-1f81-7ce2-0050fb744bd7@linux.intel.com> From: "Liu, Jing2" Message-ID: Date: Tue, 1 Jun 2021 18:24:34 +0800 User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:78.0) Gecko/20100101 Thunderbird/78.6.0 MIME-Version: 1.0 In-Reply-To: Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 8bit Content-Language: en-US Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On 5/26/2021 10:43 PM, Sean Christopherson wrote: > On Wed, May 26, 2021, Liu, Jing2 wrote: >> On 5/25/2021 5:50 AM, Sean Christopherson wrote: >>> On Sun, Feb 07, 2021, Jing Liu wrote: >>>> The static xstate buffer kvm_xsave contains the extended register >>>> states, but it is not enough for dynamic features with large state. >>>> >>>> Introduce a new capability called KVM_CAP_X86_XSAVE_EXTENSION to >>>> detect if hardware has XSAVE extension (XFD). Meanwhile, add two >>>> new ioctl interfaces to get/set the whole xstate using struct >>>> kvm_xsave_extension buffer containing both static and dynamic >>>> xfeatures. Reuse fill_xsave and load_xsave for both cases. >>>> >>>> Signed-off-by: Jing Liu >>>> --- >>>> arch/x86/include/uapi/asm/kvm.h | 5 +++ >>>> arch/x86/kvm/x86.c | 70 +++++++++++++++++++++++++-------- >>>> include/uapi/linux/kvm.h | 8 ++++ >>>> 3 files changed, 66 insertions(+), 17 deletions(-) >>>> >>>> diff --git a/arch/x86/include/uapi/asm/kvm.h b/arch/x86/include/uapi/asm/kvm.h >>>> index 89e5f3d1bba8..bf785e89a728 100644 >>>> --- a/arch/x86/include/uapi/asm/kvm.h >>>> +++ b/arch/x86/include/uapi/asm/kvm.h >>>> @@ -362,6 +362,11 @@ struct kvm_xsave { >>>> __u32 region[1024]; > Hold up a sec. How big is the AMX data? AMX tileconfig size is 64B, but tiledata size is 8K. > The existing size is 4096 bytes, not > 1024 bytes. IIRC, AMX is >4k, so we still need a new ioctl(), Yep, kvm_xsave can hold 4KB state. We need a new ioctl, holding all the states, not only AMX. And once KVM supports AMX, the size will >4096 so qemu need use kvm_xsave2 instead, otherwise, cannot save/restore whole AMX state. > but we should be > careful to mentally adjust for the __u32 when mentioning the sizes. > >>>> }; >>>> +/* for KVM_CAP_XSAVE_EXTENSION */ >>>> +struct kvm_xsave_extension { >>>> + __u32 region[3072]; >>> Fool me once, shame on you (Intel). Fool me twice, shame on me (KVM). >>> >>> As amusing as kvm_xsave_really_extended would be, the required size should be >>> discoverable, not hardcoded. >> Thanks for reviewing the patch. When looking at current kvm_xsave structure, >> I felt confusing about the static hardcoding of 1024 bytes, but failed to >> find clue for its final decision in 2010[1]. > Simplicitly and lack of foresight :-) > >> So we'd prefer to changing the way right? Please correct me if I misunderstood. > Sadly, we can't fix the existing ioctl() without breaking userspace. But for > the new ioctl(), yes, its size should not be hardcoded. > >>> Nothing prevents a hardware vendor from inventing a newfangled feature that >>> requires yet more space. As an alternative to adding a dedicated >>> capability, can we leverage GET_SUPPORTED_CPUID, leaf CPUID.0xD, >> Yes, this is a good way to avoid a dedicated capability. Thanks for the >> suggestion. Use 0xD.1.EBX for size of enabled xcr0|xss if supposing >> kvm_xsave cares both. >>> to enumerate the minimum required size and state >> For the state, an extreme case is using an old qemu as follows, but a >> new kvm with more future_featureZ supported. If hardware vendor arranges >> one by one, it's OK to use static state like X86XSaveArea(2) and >> get/set between userspace and kvm because it's non-compacted. If not, >> the state will not correct. >> So far it is OK, so I'm wondering if this would be an issue for now? > Oh, you're saying that, because kvm_xsave is non-compacted, future features may > overflow kvm_xsave simply because the architectural offset overflows 4096 bytes. > > That should be a non-issue for old KVM/kernels, since the new features shouldn't > be enabled. For new KVM, I think the right approach is to reject KVM_GET_XSAVE > and KVM_SET_XSAVE if the required size is greater than sizeof(struct kvm_xsave). > I.e. force userspace to either hide the features from the guest, or use > KVM_{G,S}ET_XSAVE2. I was considering if the order/offset of future features will impact the compatibility if it is not designed one by one. But I realized it's not an issue because there uses non-compacted format so each offset strictly refers to spec. >> X86XSaveArea2 { >>     ... >>     XSaveAVX >>     ... >>     AMX_XTILE; >>     future_featureX; >>     future_featureY; >> } >> >>> that the new ioctl() is available if the min size is greater than 1024? >>> Or is that unnecessarily convoluted... >> To enable a dynamic size kvm_xsave2(Thanks Jim's name suggestion), if things >> as follows are what we might want. >> /* for xstate large than 1024 */ >> struct kvm_xsave2 { >>     int size; // size of the whole xstate >>     void *ptr; // xstate pointer >> } >> #define KVM_GET_XSAVE2   _IOW(KVMIO,  0xa4, struct kvm_xsave2) >> >> Take @size together, so KVM need not fetch 0xd.1.ebx each time or a dedicated >> variable. > Yes, userspace needs to provide the size so that KVM doesn't unintentionally > overflow the buffer provided by userspace. We might also want to hedge by adding > a flags? Can't think of a use for it at the moment, though. > > struct kvm_xsave2 { > __u32 flags; > __u32 size; > __u8 state[0]; > }; u8 makes things simple that kvm doesn't need compute size to u32. Thanks, Jing