linux-kernel - Re: [PATCH v1 1/2] x86/kvm/fpu: Mask guest fpstate->xfeatures with guest_supported

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Message-ID: <8bf8ba96-94a8-663a-ccbf-ffeab087c370@redhat.com>
Date:   Mon, 7 Feb 2022 22:00:14 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Leonardo Bras Soares Passos <leobras@...hat.com>
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 1/2] x86/kvm/fpu: Mask guest fpstate->xfeatures with
 guest_supported_xcr0

On 2/7/22 21:24, Leonardo Bras Soares Passos wrote:
>> With this patch,
>> we have to reason about the effect of calling KVM_SET_CPUID2 twice calls
>> back to back.  I think an "&=" would be wrong in that case.
> 
> So, you suggest something like this ?
> 
> vcpu->arch.guest_fpu.fpstate->xfeatures =
>         fpu_user_cfg.default_features & vcpu->arch.guest_supported_xcr0;
> 

Yes, but you need to change user_xfeatures instead of xfeatures. 
KVM_GET_XSAVE and KVM_SET_XSAVE will take it into account automatically:

- KVM_GET_XSAVE: fpu_copy_guest_fpstate_to_uabi -> __copy_xstate_to_uabi_buf

- KVM_SET_XSAVE: fpu_copy_uabi_to_guest_fpstate -> 
copy_uabi_from_kernel_to_xstate -> copy_uabi_to_xstate -> 
validate_user_xstate_buffer

Paolo