From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id CD0143BFE3B for ; Thu, 15 Jan 2026 18:26:28 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768501590; cv=none; b=clFpTVbsyw72WfH3lVcRVf8ibhBZ26GXJu9usGMyItLzF0w+pMbeNbOhZUNhxoeC/CnmbtrZw5+chqUnOBTn5JsuUGf2zqARWPJLvDA2k+NCk8DNiPV4vSVWAOah66vc0diRiYKt/yEVwvZ3u2bZ5LjnZOs93NQ+rvcpn1J9tq4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768501590; c=relaxed/simple; bh=0w7qjlkys/t+zaa2dmz/r3JRm0vyMo31Nn/jlIfk5GQ=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=M4XQnhlZVRq3msqJdMcnQwstvOE8qjEylS4iMff9Pm9xTkmoDnT39548QJwipcDAP4IeRjU/qTsYO3syrKimkyTpeOOpXPDaFCn8N1gW8LsCKpJ/JXUu2Gk0KQFnpVqhnWC/mBfCwqRa5FkYNyjYoh24CXdY7hDO9HYRz4cT9Dk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=Eutkmsbj; dkim=pass (2048-bit key) header.d=redhat.com header.i=@redhat.com header.b=R22dN5Sv; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="Eutkmsbj"; dkim=pass (2048-bit key) header.d=redhat.com header.i=@redhat.com header.b="R22dN5Sv" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1768501587; 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: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:autocrypt:autocrypt; bh=/4PSeZvb7SjbMj4tci2c4EAiTmFPySPC3Po5tN6+TGM=; b=EutkmsbjtWpY0u/yxiZ5M5lqj78nauslOnWrE4aaeC01DwwAhWRIkIIrRfIufa3LlAaTKE dAO83dY4UFmrWwxpwXbySu2D2ed6cjK5T82uxceGKb7cdAynhs69lQzTQiTfuVUP6xWP3e +TaGio/nuoVonnHkTcodOaGD/DlT9PI= Received: from mail-ed1-f72.google.com (mail-ed1-f72.google.com [209.85.208.72]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-609-Q8cfrFngP6qkuNs4x14n-Q-1; Thu, 15 Jan 2026 13:26:26 -0500 X-MC-Unique: Q8cfrFngP6qkuNs4x14n-Q-1 X-Mimecast-MFC-AGG-ID: Q8cfrFngP6qkuNs4x14n-Q_1768501585 Received: by mail-ed1-f72.google.com with SMTP id 4fb4d7f45d1cf-64ba73e83c9so1854585a12.2 for ; Thu, 15 Jan 2026 10:26:26 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=google; t=1768501585; x=1769106385; darn=vger.kernel.org; h=content-transfer-encoding:in-reply-to:autocrypt:content-language :from:references:cc:to:subject:user-agent:mime-version:date :message-id:from:to:cc:subject:date:message-id:reply-to; bh=/4PSeZvb7SjbMj4tci2c4EAiTmFPySPC3Po5tN6+TGM=; b=R22dN5SvefN9ZXHRHXOwBBpPXYMxNzJkZCsVTXXIWihqHW+6IuftSeqAKv9kKoFF0t QOHiUWketlrtp2+zmGg9fJJWiXfyh70bZ/f0nSSi+7rAMnO/M0fGI13PURBGqYnoEJ31 8pfP4DO+EYZ9gs4yGWdz/dBceyKub8LV3TtYuP0+OFbbg4fBFaWNoRSYMZbFe8IxNLgq sv9MCYecPHMNR6kljhFffX5Z8Af/FKA+FtBuIKxf4JyW1WNCIeGzEsvFzMDbR1H1R9ic iS05pke37u9PlkBcP8+mkMJXVTdy3ekthFFpJ57AScbGzMbj23zzs1vtxm4Pr9b06mrb CYfQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1768501585; x=1769106385; h=content-transfer-encoding:in-reply-to:autocrypt:content-language :from:references:cc:to:subject:user-agent:mime-version:date :message-id:x-gm-gg:x-gm-message-state:from:to:cc:subject:date :message-id:reply-to; bh=/4PSeZvb7SjbMj4tci2c4EAiTmFPySPC3Po5tN6+TGM=; b=mznYcHqmCokVXUeKuqNxlAeaZjTXgdMIUkrep7LxSxKd/jhIrDmuCemcDeSXny3ngB b7tjHk+QSWmb8KN7JEBY9cyk79hJSGc8+sCx8JOZwqdaCgUZy861sw5ZDTIxRyqWuVpw er2Al3Y3I8nyi0v3GCQgrP/+OqthuM8kTJ7fjbI9xx8gcb34TkL+K4UiBkirTZUt9cx2 D1h4y1aatf9v59coEKRVoe/x45Xaz39E4xmuoD5FCbBDtALN2nngNvctfki1fODqElt+ gXQDq1/tHfximzFrk9X4kbIidiGhd5aQOVjFuTMz25P2b4R7hrsHIi5ZPIVrGPQlRbf7 a9TA== X-Forwarded-Encrypted: i=1; AJvYcCXFM3n6q3uPjBwTEglxDvCfJKnAC51z1tPUKfzfP/BNegHxXZU43Q17FV+6Uz5h5JjNkzYYEE94A0PLZlY=@vger.kernel.org X-Gm-Message-State: AOJu0YyDdQBUPQWaZ6bm/r5m3TiyTZrRIcR4t7AvS/DGbrzHZo39TsFc XQe/IOdbRHqWEqNy7KsUhQiR2+COAY6nFC/QNGK+0jtq34/JiCdRUMFVYXppbzi3/WvqyqHDKhW n66WA6XG979n+0LMNvViNRgb1eX6bYT9YoU20+fiZneLb6eu8SVKy9iG0/8u4TNR/TA== X-Gm-Gg: AY/fxX5T9VOaqtMLqBpcmAlPreXlDmLejNpDJdm5E3A05nuJq3etKN0d9kzfa8nKIai wWUra6MRzaUgEnt/EkJi/X4Jhkc0cDT6W/z4DFccm8v1XU4umSOgLUAWY04jQ5nmIOdhxi16mur 4P3/cgvn7b7LjOgMO40bH2XPlRuVcFRFAN1B/pL6X7P492/hcSGKSu3Kiv+A1LJ82n5GrsdxrfQ YKV5P2IaDfVU7D1BgCOW5AYMDSvkItjCiW00KfuucRojbpvhAo3+6xMG0rZ9Wmr4LKK3uI2m7oL CQjYDOC0czGMF4eiuZtR2fFls0P9AdpH2g53j4xi3y3lMybIveoqNAaRb8k65MwbPVo7zyh9c97 QwB0Zf3CfRkZMkVX0yhP97VCaU8VryRdWjUhnYx0ztXHmVTxEtOV4Egkt+rFljjIGs5dU1wNJfK hkCeDDLRSn7rc= X-Received: by 2002:a17:907:846:b0:b87:6f2:a486 with SMTP id a640c23a62f3a-b8792f79ecdmr47544666b.31.1768501585155; Thu, 15 Jan 2026 10:26:25 -0800 (PST) X-Received: by 2002:a17:907:846:b0:b87:6f2:a486 with SMTP id a640c23a62f3a-b8792f79ecdmr47542466b.31.1768501584718; Thu, 15 Jan 2026 10:26:24 -0800 (PST) Received: from [192.168.1.84] ([93.56.161.93]) by smtp.googlemail.com with ESMTPSA id a640c23a62f3a-b87959f6e53sm7681666b.47.2026.01.15.10.26.23 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 15 Jan 2026 10:26:24 -0800 (PST) Message-ID: Date: Thu, 15 Jan 2026 19:26:22 +0100 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/4] x86/fpu: Clear XSTATE_BV[i] in save state whenever XFD[i]=1 To: Dave Hansen , linux-kernel@vger.kernel.org, kvm@vger.kernel.org Cc: seanjc@google.com, x86@kernel.org, Thomas Gleixner , Borislav Petkov , "Bae, Chang Seok" References: <20260101090516.316883-1-pbonzini@redhat.com> <20260101090516.316883-2-pbonzini@redhat.com> <8ee84cb9-ef6d-43ac-b9d0-9c22e7d1ecd8@redhat.com> From: Paolo Bonzini Content-Language: en-US Autocrypt: addr=pbonzini@redhat.com; keydata= xsEhBFRCcBIBDqDGsz4K0zZun3jh+U6Z9wNGLKQ0kSFyjN38gMqU1SfP+TUNQepFHb/Gc0E2 CxXPkIBTvYY+ZPkoTh5xF9oS1jqI8iRLzouzF8yXs3QjQIZ2SfuCxSVwlV65jotcjD2FTN04 hVopm9llFijNZpVIOGUTqzM4U55sdsCcZUluWM6x4HSOdw5F5Utxfp1wOjD/v92Lrax0hjiX DResHSt48q+8FrZzY+AUbkUS+Jm34qjswdrgsC5uxeVcLkBgWLmov2kMaMROT0YmFY6A3m1S P/kXmHDXxhe23gKb3dgwxUTpENDBGcfEzrzilWueOeUWiOcWuFOed/C3SyijBx3Av/lbCsHU Vx6pMycNTdzU1BuAroB+Y3mNEuW56Yd44jlInzG2UOwt9XjjdKkJZ1g0P9dwptwLEgTEd3Fo UdhAQyRXGYO8oROiuh+RZ1lXp6AQ4ZjoyH8WLfTLf5g1EKCTc4C1sy1vQSdzIRu3rBIjAvnC tGZADei1IExLqB3uzXKzZ1BZ+Z8hnt2og9hb7H0y8diYfEk2w3R7wEr+Ehk5NQsT2MPI2QBd wEv1/Aj1DgUHZAHzG1QN9S8wNWQ6K9DqHZTBnI1hUlkp22zCSHK/6FwUCuYp1zcAEQEAAc0j UGFvbG8gQm9uemluaSA8cGJvbnppbmlAcmVkaGF0LmNvbT7CwU0EEwECACMFAlRCcBICGwMH CwkIBwMCAQYVCAIJCgsEFgIDAQIeAQIXgAAKCRB+FRAMzTZpsbceDp9IIN6BIA0Ol7MoB15E 11kRz/ewzryFY54tQlMnd4xxfH8MTQ/mm9I482YoSwPMdcWFAKnUX6Yo30tbLiNB8hzaHeRj jx12K+ptqYbg+cevgOtbLAlL9kNgLLcsGqC2829jBCUTVeMSZDrzS97ole/YEez2qFpPnTV0 VrRWClWVfYh+JfzpXmgyhbkuwUxNFk421s4Ajp3d8nPPFUGgBG5HOxzkAm7xb1cjAuJ+oi/K CHfkuN+fLZl/u3E/fw7vvOESApLU5o0icVXeakfSz0LsygEnekDbxPnE5af/9FEkXJD5EoYG SEahaEtgNrR4qsyxyAGYgZlS70vkSSYJ+iT2rrwEiDlo31MzRo6Ba2FfHBSJ7lcYdPT7bbk9 AO3hlNMhNdUhoQv7M5HsnqZ6unvSHOKmReNaS9egAGdRN0/GPDWr9wroyJ65ZNQsHl9nXBqE AukZNr5oJO5vxrYiAuuTSd6UI/xFkjtkzltG3mw5ao2bBpk/V/YuePrJsnPFHG7NhizrxttB nTuOSCMo45pfHQ+XYd5K1+Cv/NzZFNWscm5htJ0HznY+oOsZvHTyGz3v91pn51dkRYN0otqr bQ4tlFFuVjArBZcapSIe6NV8C4cEiSTOwE0EVEJx7gEIAMeHcVzuv2bp9HlWDp6+RkZe+vtl KwAHplb/WH59j2wyG8V6i33+6MlSSJMOFnYUCCL77bucx9uImI5nX24PIlqT+zasVEEVGSRF m8dgkcJDB7Tps0IkNrUi4yof3B3shR+vMY3i3Ip0e41zKx0CvlAhMOo6otaHmcxr35sWq1Jk tLkbn3wG+fPQCVudJJECvVQ//UAthSSEklA50QtD2sBkmQ14ZryEyTHQ+E42K3j2IUmOLriF dNr9NvE1QGmGyIcbw2NIVEBOK/GWxkS5+dmxM2iD4Jdaf2nSn3jlHjEXoPwpMs0KZsgdU0pP JQzMUMwmB1wM8JxovFlPYrhNT9MAEQEAAcLBMwQYAQIACQUCVEJx7gIbDAAKCRB+FRAMzTZp sadRDqCctLmYICZu4GSnie4lKXl+HqlLanpVMOoFNnWs9oRP47MbE2wv8OaYh5pNR9VVgyhD OG0AU7oidG36OeUlrFDTfnPYYSF/mPCxHttosyt8O5kabxnIPv2URuAxDByz+iVbL+RjKaGM GDph56ZTswlx75nZVtIukqzLAQ5fa8OALSGum0cFi4ptZUOhDNz1onz61klD6z3MODi0sBZN Aj6guB2L/+2ZwElZEeRBERRd/uommlYuToAXfNRdUwrwl9gRMiA0WSyTb190zneRRDfpSK5d usXnM/O+kr3Dm+Ui+UioPf6wgbn3T0o6I5BhVhs4h4hWmIW7iNhPjX1iybXfmb1gAFfjtHfL xRUr64svXpyfJMScIQtBAm0ihWPltXkyITA92ngCmPdHa6M1hMh4RDX+Jf1fiWubzp1voAg0 JBrdmNZSQDz0iKmSrx8xkoXYfA3bgtFN8WJH2xgFL28XnqY4M6dLhJwV3z08tPSRqYFm4NMP dRsn0/7oymhneL8RthIvjDDQ5ktUjMe8LtHr70OZE/TT88qvEdhiIVUogHdo4qBrk41+gGQh b906Dudw5YhTJFU3nC6bbF2nrLlB4C/XSiH76ZvqzV0Z/cAMBo5NF/w= In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 1/15/26 19:19, Dave Hansen wrote: > Is there anything preventing the kernel_fpu_begin() interrupt from > happening a little later, say: > > XFD[18]=0 > ... > XFD[18]=1 > # Interrupt (that does XSAVE) > XRSTOR (no #NM) > > In that case, the XSAVE in kernel_fpu_begin() "operates as if XINUSE[i] > = 0" and would set XFEATURES[18]=0; it would save the component as being > in its init state. The later XRSTOR would obviously restore state 18 to > its init state. Yes, absolutely, and the fact that the race window is so small is why this issue stayed undetected for years. In fact, consider that XFD becomes a pass-through MSR after the first write, at which point there's on race window at all---XFD[18] will be 1 if that's the guest value and the state will be destroyed. I only mentioned SMIs as a way for this to happen on bare metal, i.e. without KVM involvement at all (though for dual-monitor treatment virtualization _is_ involved). > That's a long-winded way of saying I think I agree with the patch. It > destroys the state a bit more aggressively but it doesn't do anything _new_. Thanks. :) > What would folks think about making the SDM language stronger, or at > least explicitly adding the language that setting XFD[i]=1 can lead to > XINUSE[i] going from 1=>0. Kinda like the language that's already in > "XRSTOR and the Init and Modified Optimizations", but specific to XFD: > > If XFD[i] = 1 and XINUSE[i] = 1, state component i may be > tracked as init; XINUSE[i] may be set to 0. > > That would make it consistent with the KVM behavior. It might also give > the CPU folks some additional wiggle room for new behavior. Yes, absolutely. I think any other hypervisor may want to do the same, to avoid save/restores of tile data to when guest XFD[18]=1 (and to avoid unnecessary clearing of XFD, just for the sake of storing tile data that is most likely unused). Paolo