From:   "Tian, Kevin" <kevin.tian@intel.com>
To:     "Christopherson,, Sean" <seanjc@google.com>,
        "Liu, Jing2" <jing2.liu@intel.com>
CC:     "x86@kernel.org" <x86@kernel.org>,
        "kvm@vger.kernel.org" <kvm@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "linux-doc@vger.kernel.org" <linux-doc@vger.kernel.org>,
        "linux-kselftest@vger.kernel.org" <linux-kselftest@vger.kernel.org>,
        "tglx@linutronix.de" <tglx@linutronix.de>,
        "mingo@redhat.com" <mingo@redhat.com>,
        "bp@alien8.de" <bp@alien8.de>,
        "dave.hansen@linux.intel.com" <dave.hansen@linux.intel.com>,
        "pbonzini@redhat.com" <pbonzini@redhat.com>,
        "corbet@lwn.net" <corbet@lwn.net>,
        "shuah@kernel.org" <shuah@kernel.org>,
        "Nakajima, Jun" <jun.nakajima@intel.com>,
        "jing2.liu@linux.intel.com" <jing2.liu@linux.intel.com>,
        "Zeng, Guang" <guang.zeng@intel.com>,
        "Wang, Wei W" <wei.w.wang@intel.com>,
        "Zhong, Yang" <yang.zhong@intel.com>
Subject: RE: [PATCH v3 13/22] kvm: x86: Intercept #NM for saving IA32_XFD_ERR
Thread-Topic: [PATCH v3 13/22] kvm: x86: Intercept #NM for saving IA32_XFD_ERR
Thread-Index: AQHX9zE1lVRiv1UsgEq40a+CF3C7OqxIomKAgACA4JA=
Date:   Wed, 29 Dec 2021 08:13:54 +0000
Message-ID: <BN9PR11MB52762B6AD795B6376F6474588C449@BN9PR11MB5276.namprd11.prod.outlook.com>
References: <20211222124052.644626-1-jing2.liu@intel.com>
 <20211222124052.644626-14-jing2.liu@intel.com> <YcunSb52LlGKT7dC@google.com>
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> From: Sean Christopherson <seanjc@google.com>
> Sent: Wednesday, December 29, 2021 8:10 AM
>
> But why even bother with an extra flag?  Can't
> vmx_update_exception_bitmap() get
> the guest's MSR_IA32_XFD value and intercept #NM accordingly?  Then you
> could
> even handle this fully in kvm_set_msr_common(), e.g.
>=20
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 2c9606380bca..c6c936d2b298 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -3704,6 +3704,8 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu,
> struct msr_data *msr_info)
>                         return 1;
>=20
>                 fpu_update_guest_xfd(&vcpu->arch.guest_fpu, data);
> +               /* Blah blah blah blah */
> +               static_call(kvm_x86_update_exception_bitmap)(vcpu);
>                 break;

btw follow Paolo's suggestion here:

https://lore.kernel.org/all/6e95b6f7-44dc-7e48-4e6e-81cf85fc11c6@redhat.com=
/

He prefers to disabling xfd interception in vmx specific code instead of
introducing a new callback to be called in common code. Since we want
to always trap #NM after xfd interception is disabled (which is done
after calling the msr common code), it also reads cleaner to call
update_exception_bitmap() from vmx.c instead of here.

Thanks
Kevin