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([2001:b07:6468:f312:8ca6:a836:a237:fed1]) by smtp.googlemail.com with ESMTPSA id u26-20020a05640207da00b00419a14928e5sm5270689edy.28.2022.03.28.10.12.46 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 28 Mar 2022 10:12:47 -0700 (PDT) Message-ID: Date: Mon, 28 Mar 2022 19:12:45 +0200 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.4.0 Subject: Re: [PATCH v4 2/6] KVM: x86: nSVM: implement nested LBR virtualization Content-Language: en-US To: Maxim Levitsky , kvm@vger.kernel.org Cc: Ingo Molnar , Borislav Petkov , Sean Christopherson , Vitaly Kuznetsov , "H. Peter Anvin" , Joerg Roedel , linux-kernel@vger.kernel.org, Thomas Gleixner , Jim Mattson , x86@kernel.org, Dave Hansen , Wanpeng Li References: <20220322174050.241850-1-mlevitsk@redhat.com> <20220322174050.241850-3-mlevitsk@redhat.com> <612b6524258b949e381efec12d85b4e82be53308.camel@redhat.com> From: Paolo Bonzini In-Reply-To: <612b6524258b949e381efec12d85b4e82be53308.camel@redhat.com> Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 3/27/22 17:12, Maxim Levitsky wrote: > - with LBR virtualization supported, the guest can set this msr to any value > as long as it doesn't set reserved bits and then read back the written value, > but it is not used by the CPU, unless LBR bit is set in MSR_IA32_DEBUGCTLMSR, > because only then LBR virtualization is enabled, which makes the CPU > load the guest value on VM entry. > > This means that MSR_IA32_DEBUGCTLMSR.BTF will magically start working when > MSR_IA32_DEBUGCTLMSR.LBR is set as well, and will not work otherwise. That can be fixed by context-switching DEBUGCTLMSR by hand when LBR=0 && BTF=1. Would you like to give it a shot? Paolo