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From: David Matlack <dmatlack@google.com>
Date: Wed, 16 Mar 2016 10:11:24 -0700
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Subject: Re: [PATCH 0/1] KVM: x86: using the fpu in interrupt context with a
 guest's xcr0
To: Andy Lutomirski <luto@amacapital.net>
Cc: Xiao Guangrong <guangrong.xiao@linux.intel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        X86 ML <x86@kernel.org>, kvm list <kvm@vger.kernel.org>,
        Paolo Bonzini <pbonzini@redhat.com>, Ingo Molnar <mingo@redhat.com>,
        Andy Lutomirski <luto@kernel.org>, "H. Peter Anvin" <hpa@zytor.com>,
        Eric Northup <digitaleric@google.com>
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On Tue, Mar 15, 2016 at 8:48 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>
> Why is it safe to rely on interrupted_kernel_fpu_idle?  That function
> is for interrupts, but is there any reason that KVM can't be preempted
> (or explicitly schedule) with XCR0 having some funny value?

KVM restores the host's xcr0 in the sched-out preempt notifier and
prior to returning to userspace.