From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from mgamail.intel.com (mgamail.intel.com [192.198.163.17]) (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 A4FF231D382; Wed, 10 Sep 2025 13:46:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=192.198.163.17 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757511972; cv=none; b=aObO7u6ybh+GvPepPMAo2cTJDBJmFE6l0DRN8ou0nKtbPxbl8UgU8kjZKJVMz0jVEEahjkNVB7Rr+L3gy1eCmYfSOl9JB4SuhNkmig//tHReliaCbhjrQkw83a+uyV1m3LvGAkOSn4fUHzNJwTnbgteXb7PS/phZQEWgZLXkuL8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1757511972; c=relaxed/simple; bh=z11WiJxFvJ2XbIvM8eaZyqRCuCpRYVlpJGYWLTsQxbs=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=Djg6d3DMHsB8PeaibXGkHhh4YsrRs7p1FaC1AecLv6u2HJqy8GAmb9FfEVzSPTJNnqa2b7c+REL6WlXBWdWkEtEwq+z2DGTUJmZuB7dtkc67LcascgbVRPh0mfG54c9Wsb49R6UW7ARWEaS33YR9N31TQEBXVkfi6FcsS9zLctU= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=intel.com; spf=pass smtp.mailfrom=intel.com; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b=L/Bdfip7; arc=none smtp.client-ip=192.198.163.17 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=intel.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=intel.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=intel.com header.i=@intel.com header.b="L/Bdfip7" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1757511971; x=1789047971; h=message-id:date:mime-version:subject:to:cc:references: from:in-reply-to:content-transfer-encoding; bh=z11WiJxFvJ2XbIvM8eaZyqRCuCpRYVlpJGYWLTsQxbs=; b=L/Bdfip7FQS4VJ9UZ/Bd7hE42BKTNNyNmPJMxZsMB0k2H8EGpXq4coaN +bRbActK84JYz9TOs9RVlCsmOVdSQbvi0QjdiM7d3fB7mgcjtDEh3BI5L VsV6zuVbPbHq563vCKKZ8y4RA8PbpNH11cTQL6XD7+kztzcS7Ar6631hh Hu8Xpp+qpERaHXPSfLCxx/YnWTADYaAveXU5zLoQtl3b3tT3sDNYUn9Hz QTmJb01CSj8aN7SIDrW7dNZtX07A45ih7KJOUahugC52a9iO/gviZYAGG hnpYHE7hDIC10pCHjUtHLYWXBHnL8/hT4CvW722WDN3u1ePCU+OJxDfir Q==; X-CSE-ConnectionGUID: tJKMwhlOQwOfImdke81pxg== X-CSE-MsgGUID: dZZBWBXBR9mBLy/fYaBuXg== X-IronPort-AV: E=McAfee;i="6800,10657,11549"; a="59759702" X-IronPort-AV: E=Sophos;i="6.18,254,1751266800"; d="scan'208";a="59759702" Received: from fmviesa008.fm.intel.com ([10.60.135.148]) by fmvoesa111.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 10 Sep 2025 06:46:10 -0700 X-CSE-ConnectionGUID: rwNJt1VpTB+PuFa4w9xoUA== X-CSE-MsgGUID: 3LYFixSSTdSwYpPB536a0Q== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.18,254,1751266800"; d="scan'208";a="173777182" Received: from xiaoyaol-hp-g830.ccr.corp.intel.com (HELO [10.124.238.14]) ([10.124.238.14]) by fmviesa008-auth.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 10 Sep 2025 06:46:04 -0700 Message-ID: <5077c390-1211-42fc-b753-2a23187cf8ca@intel.com> Date: Wed, 10 Sep 2025 21:46:01 +0800 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 v14 06/22] KVM: x86: Load guest FPU state when access XSAVE-managed MSRs To: Chao Gao Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, acme@redhat.com, bp@alien8.de, dave.hansen@linux.intel.com, hpa@zytor.com, john.allen@amd.com, mingo@kernel.org, mingo@redhat.com, minipli@grsecurity.net, mlevitsk@redhat.com, namhyung@kernel.org, pbonzini@redhat.com, prsampat@amd.com, rick.p.edgecombe@intel.com, seanjc@google.com, shuah@kernel.org, tglx@linutronix.de, weijiang.yang@intel.com, x86@kernel.org, xin@zytor.com References: <20250909093953.202028-1-chao.gao@intel.com> <20250909093953.202028-7-chao.gao@intel.com> Content-Language: en-US From: Xiaoyao Li In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit On 9/10/2025 7:18 PM, Chao Gao wrote: > On Wed, Sep 10, 2025 at 05:37:50PM +0800, Xiaoyao Li wrote: >> On 9/9/2025 5:39 PM, Chao Gao wrote: >>> From: Sean Christopherson >>> >>> Load the guest's FPU state if userspace is accessing MSRs whose values >>> are managed by XSAVES. Introduce two helpers, kvm_{get,set}_xstate_msr(), >>> to facilitate access to such kind of MSRs. >>> >>> If MSRs supported in kvm_caps.supported_xss are passed through to guest, >>> the guest MSRs are swapped with host's before vCPU exits to userspace and >>> after it reenters kernel before next VM-entry. >>> >>> Because the modified code is also used for the KVM_GET_MSRS device ioctl(), >>> explicitly check @vcpu is non-null before attempting to load guest state. >>> The XSAVE-managed MSRs cannot be retrieved via the device ioctl() without >>> loading guest FPU state (which doesn't exist). >>> >>> Note that guest_cpuid_has() is not queried as host userspace is allowed to >>> access MSRs that have not been exposed to the guest, e.g. it might do >>> KVM_SET_MSRS prior to KVM_SET_CPUID2. > > ... > >>> + bool fpu_loaded = false; >>> int i; >>> - for (i = 0; i < msrs->nmsrs; ++i) >>> + for (i = 0; i < msrs->nmsrs; ++i) { >>> + /* >>> + * If userspace is accessing one or more XSTATE-managed MSRs, >>> + * temporarily load the guest's FPU state so that the guest's >>> + * MSR value(s) is resident in hardware, i.e. so that KVM can >>> + * get/set the MSR via RDMSR/WRMSR. >>> + */ >>> + if (vcpu && !fpu_loaded && kvm_caps.supported_xss && >> >> why not check vcpu->arch.guest_supported_xss? > > Looks like Sean anticipated someone would ask this question. here it determines whether to call kvm_load_guest_fpu(). - based on kvm_caps.supported_xss, it will always load guest fpu. - based on vcpu->arch.guest_supported_xss, it depends on whether userspace calls KVM_SET_CPUID2 and whether it enables any XSS feature. So the difference is when no XSS feature is enabled for the VM. In this case, if checking vcpu->arch.guest_supported_xss, it will skip kvm_load_guest_fpu(). And it will result in GET_MSR gets usrerspace's value and SET_MSR changes userspace's value, when MSR access is eventually allowed in later do_msr() callback. Is my understanding correctly?