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([2001:b07:6468:f312:1c48:1dd8:fe63:e3da]) by smtp.gmail.com with ESMTPSA id dm23sm2518052edb.0.2020.05.20.14.05.17 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 20 May 2020 14:05:17 -0700 (PDT) Subject: Re: [PATCH 2/2] kvm/x86: don't expose MSR_IA32_UMWAIT_CONTROL unconditionally To: Maxim Levitsky , kvm@vger.kernel.org Cc: linux-kernel@vger.kernel.org References: <20200520160740.6144-1-mlevitsk@redhat.com> <20200520160740.6144-3-mlevitsk@redhat.com> From: Paolo Bonzini Message-ID: Date: Wed, 20 May 2020 23:05:17 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <20200520160740.6144-3-mlevitsk@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: kvm-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org On 20/05/20 18:07, Maxim Levitsky wrote: > This msr is only available when the host supports WAITPKG feature. > > This breaks a nested guest, if the L1 hypervisor is set to ignore > unknown msrs, because the only other safety check that the > kernel does is that it attempts to read the msr and > rejects it if it gets an exception. > > Fixes: 6e3ba4abce KVM: vmx: Emulate MSR IA32_UMWAIT_CONTROL > > Signed-off-by: Maxim Levitsky > --- > arch/x86/kvm/x86.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index fe3a24fd6b263..9c507b32b1b77 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -5314,6 +5314,10 @@ static void kvm_init_msr_list(void) > if (msrs_to_save_all[i] - MSR_ARCH_PERFMON_EVENTSEL0 >= > min(INTEL_PMC_MAX_GENERIC, x86_pmu.num_counters_gp)) > continue; > + break; > + case MSR_IA32_UMWAIT_CONTROL: > + if (!kvm_cpu_cap_has(X86_FEATURE_WAITPKG)) > + continue; > default: > break; > } The patch is correct, and matches what is done for the other entries of msrs_to_save_all. However, while looking at it I noticed that X86_FEATURE_WAITPKG is actually never added, and that is because it was also not added to the supported CPUID in commit e69e72faa3a0 ("KVM: x86: Add support for user wait instructions", 2019-09-24), which was before the kvm_cpu_cap mechanism was added. So while at it you should also fix that. The right way to do that is to add a if (vmx_waitpkg_supported()) kvm_cpu_cap_check_and_set(X86_FEATURE_WAITPKG); in vmx_set_cpu_caps. Thanks, Paolo