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Message-ID: <6bee793c-f7fc-2ede-0405-7a5d7968b175@redhat.com>
Date:   Mon, 14 Feb 2022 10:56:22 +0100
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Subject: Re: [PATCH v3 1/1] x86/kvm/fpu: Mask guest fpstate->xfeatures with
 guest_supported_xcr0
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To:     David Edmondson <david.edmondson@oracle.com>
Cc:     Leonardo Bras <leobras@redhat.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>, x86@kernel.org,
        "H. Peter Anvin" <hpa@zytor.com>,
        Sean Christopherson <seanjc@google.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        "Chang S. Bae" <chang.seok.bae@intel.com>,
        Yang Zhong <yang.zhong@intel.com>,
        Andy Lutomirski <luto@kernel.org>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org
References: <20220211060742.34083-1-leobras@redhat.com>
 <5fd84e2f-8ebc-9a4c-64bf-8d6a2c146629@redhat.com>
 <cunsfslpyvh.fsf@oracle.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <cunsfslpyvh.fsf@oracle.com>
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On 2/14/22 10:43, David Edmondson wrote:
> Sorry if this is a daft question:
> 
> In what situations will there be bits set in
> vcpu->arch.guest_supported_xcr0 that are not set in
> vcpu->arch.guest_fpu.fpstate->xfeatures ?
> 
> guest_supported_xcr0 is filtered based on supported_xcr0, which I would
> expect to weed out all bits that are not set in ->xfeatures.

Good point, so we can do just

	vcpu->arch.guest_fpu.fpstate->user_xfeatures =
		vcpu->arch.guest_supported_xcr0;

On top of this patch, we can even replace vcpu->arch.guest_supported_xcr0
with vcpu->arch.guest_fpu.fpstate->user_xfeatures.  Probably with local
variables or wrapper functions though, so as to keep the code readable.
For example:

static inline u64 kvm_guest_supported_xfd()
{
	u64 guest_supported_xcr0 = vcpu->arch.guest_fpu.fpstate->user_xfeatures;

	return guest_supported_xcr0 & XFEATURE_MASK_USER_DYNAMIC;
}

Also, already in this patch fpstate_realloc should do

         newfps->user_xfeatures = curfps->user_xfeatures | xfeatures;

only if !guest_fpu.  In other words, the user_xfeatures of the guest FPU
should be controlled exclusively by KVM_SET_CPUID2.

Thanks,

Paolo