linux-kernel - RE: [PATCH v3 13/22] kvm: x86: Intercept #NM for saving IA32_XFD

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Message-ID: <BN9PR11MB52762B6AD795B6376F6474588C449@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Wed, 29 Dec 2021 08:13:54 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     "Christopherson,, Sean" <seanjc@...gle.com>,
        "Liu, Jing2" <jing2.liu@...el.com>
CC:     "x86@...nel.org" <x86@...nel.org>,
        "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-doc@...r.kernel.org" <linux-doc@...r.kernel.org>,
        "linux-kselftest@...r.kernel.org" <linux-kselftest@...r.kernel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "mingo@...hat.com" <mingo@...hat.com>,
        "bp@...en8.de" <bp@...en8.de>,
        "dave.hansen@...ux.intel.com" <dave.hansen@...ux.intel.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "corbet@....net" <corbet@....net>,
        "shuah@...nel.org" <shuah@...nel.org>,
        "Nakajima, Jun" <jun.nakajima@...el.com>,
        "jing2.liu@...ux.intel.com" <jing2.liu@...ux.intel.com>,
        "Zeng, Guang" <guang.zeng@...el.com>,
        "Wang, Wei W" <wei.w.wang@...el.com>,
        "Zhong, Yang" <yang.zhong@...el.com>
Subject: RE: [PATCH v3 13/22] kvm: x86: Intercept #NM for saving IA32_XFD_ERR

> From: Sean Christopherson <seanjc@...gle.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.
> 
> 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;
> 
>                 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