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Message-ID: <6bee793c-f7fc-2ede-0405-7a5d7968b175@redhat.com>
Date: Mon, 14 Feb 2022 10:56:22 +0100
From: Paolo Bonzini <pbonzini@...hat.com>
To: David Edmondson <david.edmondson@...cle.com>
Cc: Leonardo Bras <leobras@...hat.com>,
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>,
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>,
"Chang S. Bae" <chang.seok.bae@...el.com>,
Yang Zhong <yang.zhong@...el.com>,
Andy Lutomirski <luto@...nel.org>,
linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v3 1/1] x86/kvm/fpu: Mask guest fpstate->xfeatures with
guest_supported_xcr0
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
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