From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.133.124]) (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 007FC3B95E6 for ; Thu, 15 Jan 2026 16:22:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.133.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768494159; cv=none; b=OG2OvGhp5TT8GfmWOt9jLrvp0g35/sDtcowwcexVoQr+qbjG2Rk3zoExYHCjkvoKz2NNERuBhos7WY1TEnBVS4pvKBqlIdC77XpNtfF3clSj6w5Mgzo27HoAYLCNWjDrWxzE4SGXjJJ1yX55looEc3ww1buKsL/iLbehXeJqRz4= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1768494159; c=relaxed/simple; bh=vW+wPZESCHcmV195EEVRxrooQk0oPEIVKKiKK3q5jlo=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=EBtWhaWMDDQSFSQhL2/YlgukA9v0tfs+L34vLIXb0MIib0/IpKEtgwwcg6ZOaderCDTibGSz7/S3hKB4spEL5ser1nDUJ5U6O4DzpaKjixhEmY7YkPNGJUAYeJ4W4bZBA+LkhDXfgDyvI25/5Zv969c3csv6Wf2F9BVB0Qh70XU= 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=eS+Lgilq; dkim=pass (2048-bit key) header.d=redhat.com header.i=@redhat.com header.b=raRg+2z+; arc=none smtp.client-ip=170.10.133.124 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="eS+Lgilq"; dkim=pass (2048-bit key) header.d=redhat.com header.i=@redhat.com header.b="raRg+2z+" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1768494155; 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=AGtT/c29PE+aFinqeoDHBaYGfiaOkvbQ97RcucMoLU0=; b=eS+LgilqUH5/E2LvbydOBnLdjt4YsMHuLjF1DPDfDoahhvrtnC8XgeC4e5S/sQ8u3CU0mk OLy6vco1cYaTGRNEHjmBfMI710eR0d8ltdlhwVuPZsykDKCao3o8cN662oZH/a/qHacKYO GvpeEr+LsDQcarELshsuUf8aiTx30yw= Received: from mail-ej1-f69.google.com (mail-ej1-f69.google.com [209.85.218.69]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-696-2ZcVyG7CMuyJL6j0mnEagw-1; Thu, 15 Jan 2026 11:22:34 -0500 X-MC-Unique: 2ZcVyG7CMuyJL6j0mnEagw-1 X-Mimecast-MFC-AGG-ID: 2ZcVyG7CMuyJL6j0mnEagw_1768494153 Received: by mail-ej1-f69.google.com with SMTP id a640c23a62f3a-b83c3dd2092so178479666b.1 for ; Thu, 15 Jan 2026 08:22:33 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=google; t=1768494153; x=1769098953; 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=AGtT/c29PE+aFinqeoDHBaYGfiaOkvbQ97RcucMoLU0=; b=raRg+2z+iwJVY995rgZjeCMR8ILR4G4+anYLNKhA1s70DYlsOmc4dgR+ZUgWnA0iYT TPww12PrNNAlSY3TjoKGcFPs9O5nojL9xxK4XdTmt77bjZ6jMq56LwJoIr4KqKD9Kepz /9pnliv/Yjm09qoNadvHrr34/A0Wo5eZHO8rGgR8Z4kOhGlIKA17IsoueCHzzD9Vy9u3 mxaTVSPcwHXwHFdPn/P2q0vs/NGqLLYPYkIS7/tgEvnU6KUQSWzYOKJrRKn6WdCuVr5a mWjhM815GoXyZuFEp0mbDUELiRRmUobTiaY/my7oO9KUg73e/vxyUF52OkVHH/T1fGP5 w5Nw== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1768494153; x=1769098953; 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=AGtT/c29PE+aFinqeoDHBaYGfiaOkvbQ97RcucMoLU0=; b=RurgvzNdwyDlBZi+i2emMH7MQwHC42VaojUogCnXGNZYvbm10DUm69qthHwPZHqvbf kY5r23XL+ByeqmkpG/cIi88rU7YTjDd3u2JBNx804RwLUq4qiBGuiYOHJseVaWilZmqY ETpEOV493Xn8REc6mzTDSBelb8qXOH0Qm+BMsbnx/gqSrtGYAL3Ta/tMfwO9pX2b64Zl VqFNhPlQgcFBb+CN+SCTPpK0vz+sZ7ORuv5ZtmcdJI95XuGEPX8T103iLlzGId/xV7kg ylOXxX5fLbXMVP0W0VJRP+O3vnf/MfSL0Wpp8sF2Ir2xVLkCN/SKJVf9HsEIyhYlNxRj JJRQ== X-Forwarded-Encrypted: i=1; AJvYcCVjNpYrmiJYc1oaO8GqN7qNeoez4GoCyAZmMDSzfL0VRcDQwkIiEk3S8B8BYv1eDusMeazCyrvgwUgSjgY=@vger.kernel.org X-Gm-Message-State: AOJu0YwPp4+/tI/hxk2SfgEeMGlVi8wvALb/lStsVp5AbmWssKzZ4j+z CP+jbVTlbh3Tkc3O6TDleuG1duw5cigX/zsXk0nRt38DQmjW0FMT8D2RiriMzQqpWhk05Cm5Z6+ 95v63gU7oLqRM9jhkpRjTjhw7VSpsRZYxpWyCn9kKpv1R76irMoHnO/bJiQ8DDLErYg== X-Gm-Gg: AY/fxX7hyk3YuRkYYoO/pHZKadJvvHc8Bmph2bIuw73xvNmpOVZLNKYOS3GF8pnQ06x ictwQ5uo85TXCqVw6T0xNlPFFSqe/hkv2HXSla0zLeJtMVVirUorKVEuZKBSu/X2N33mAzc9AGR yJYIA9u8eOvo2gV1uwE0zLglQ094gJaCgwEFYTVayQ8mAsmNiTqmcvrd/ToTrEM0bPNgWNvz5KU 0Q8F9ge/KrrD3FlD+D5UCPhtZdhgfHtdl1PFAXoCW0wyvSPEkKI9SOEiVhK9TUophm0uS6uPl4d zvfTP578HY/72rdC8j965urLLCvxbQ+Jzbsj3E1gwWZdhDXhgWjJrt/QiQxgtD9qFIYjxjFcFcg ZPb1xYLTtV96+NuJZk95/vB/G49tvZjN3bcDJrECJBClx5qUjcA+ugBY3p83rRy56IoU/zDsmgQ YLcXFDK+YwQ3E= X-Received: by 2002:a17:907:97d5:b0:b87:6d6b:1353 with SMTP id a640c23a62f3a-b8792f9e523mr13090766b.28.1768494152761; Thu, 15 Jan 2026 08:22:32 -0800 (PST) X-Received: by 2002:a17:907:97d5:b0:b87:6d6b:1353 with SMTP id a640c23a62f3a-b8792f9e523mr13089366b.28.1768494152392; Thu, 15 Jan 2026 08:22:32 -0800 (PST) Received: from [192.168.1.84] ([93.56.161.93]) by smtp.googlemail.com with ESMTPSA id a640c23a62f3a-b8793c6bbf7sm18266b.19.2026.01.15.08.22.31 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 15 Jan 2026 08:22:31 -0800 (PST) Message-ID: <8ee84cb9-ef6d-43ac-b9d0-9c22e7d1ecd8@redhat.com> Date: Thu, 15 Jan 2026 17:22:30 +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, stable@vger.kernel.org References: <20260101090516.316883-1-pbonzini@redhat.com> <20260101090516.316883-2-pbonzini@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 16:54, Dave Hansen wrote: > On 1/1/26 01:05, Paolo Bonzini wrote: >> When loading guest XSAVE state via KVM_SET_XSAVE, and when updating XFD in >> response to a guest WRMSR, clear XFD-disabled features in the saved (or to >> be restored) XSTATE_BV to ensure KVM doesn't attempt to load state for >> features that are disabled via the guest's XFD. Because the kernel >> executes XRSTOR with the guest's XFD, saving XSTATE_BV[i]=1 with XFD[i]=1 >> will cause XRSTOR to #NM and panic the kernel. > > It would be really nice to see the actual ordering of events here. What > order do the KVM_SET_XSAVE, XFD[$FOO]=1 and kernel_fpu_begin() have to > happen in to trigger this? The problematic case is described a couple paragraphs below: "This can happen if the guest executes WRMSR(MSR_IA32_XFD) to set XFD[18] = 1, and a host IRQ triggers kernel_fpu_begin() prior to the vmexit handler's call to fpu_update_guest_xfd()." Or more in detail: Guest running with MSR_IA32_XFD = 0 WRMSR(MSR_IA32_XFD) vmexit Host: enable IRQ interrupt handler kernel_fpu_begin() -> sets TIF_NEED_FPU_LOAD XSAVE -> stores XINUSE[18] = 1 ... kernel_fpu_end() handle vmexit fpu_update_guest_xfd() -> XFD[18] = 1 reenter guest fpu_swap_kvm_fpstate() XRSTOR -> XINUSE[18] = 1 && XFD[18] = 1 -> #NM and boom With the patch, fpu_update_guest_xfd() sees TIF_NEED_FPU_LOAD set and clears the bit from xinuse. Paolo