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Thu, 1 Feb 2018 13:10:00 -0500 Subject: Re: [PATCH] KVM: x86: fix backward migration with async_PF To: =?UTF-8?B?UmFkaW0gS3LEjW3DocWZ?= , linux-kernel@vger.kernel.org, kvm@vger.kernel.org Cc: Wanpeng Li , stable@vger.kernel.org References: <20180201175022.25011-1-rkrcmar@redhat.com> From: Paolo Bonzini Message-ID: Date: Thu, 1 Feb 2018 13:09:56 -0500 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.5.2 MIME-Version: 1.0 In-Reply-To: <20180201175022.25011-1-rkrcmar@redhat.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: stable-owner@vger.kernel.org X-Mailing-List: stable@vger.kernel.org X-getmail-retrieved-from-mailbox: INBOX X-Mailing-List: linux-kernel@vger.kernel.org List-ID: On 01/02/2018 12:50, Radim Krčmář wrote: > Guests on new hypersiors might set KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT > bit when enabling async_PF, but this bit is reserved on old hypervisors, > which results in a failure upon migration. > > Guests at least expect that KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT might not > be present when booting, so we allow userspace to handle migration > compatibility by adding a KVM CPUID flag that determines the presence of > KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT. > > Fixes: 52a5c155cf79 ("KVM: async_pf: Let guest support delivery of async_pf from guest mode") > Cc: > Signed-off-by: Radim Krčmář This has to be documented in Documentation/virtual/kvm/cpuid.txt. > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c > index 4c3103f449a3..c16740a06f0c 100644 > --- a/arch/x86/kvm/x86.c > +++ b/arch/x86/kvm/x86.c > @@ -2139,8 +2139,10 @@ static int kvm_pv_enable_async_pf(struct kvm_vcpu *vcpu, u64 data) > { > gpa_t gpa = data & ~0x3f; > > - /* Bits 3:5 are reserved, Should be zero */ > - if (data & 0x38) > + /* Bits 3:5 are reserved, Should be zero. */ > + if (data & 0x38 || > + (data & KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT && > + !guest_kvm_cpuid_has(vcpu, KVM_FEATURE_ASYNC_PF_VMEXIT))) > return 1; > > vcpu->arch.apf.msr_val = data; > This check will break migration if the source guest and host both have the recent kernels which support KVM_ASYNC_PF_DELIVERY_AS_PF_VMEXIT, so I am not sure about it. Otherwise, the patch is okay! Thanks, Paolo