linux-kernel - Re: [PATCH 0/1] KVM: x86: using the fpu in interrupt context with a guest's xcr0

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Message-ID: <CALzav=f80zQBR5w_=r7K9B3BZOuTfNmXnBv9JztatQMQQn3sTg@mail.gmail.com>
Date:	Wed, 16 Mar 2016 10:11:24 -0700
From:	David Matlack <dmatlack@...gle.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, kvm list <kvm@...r.kernel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	Andy Lutomirski <luto@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Eric Northup <digitaleric@...gle.com>
Subject: Re: [PATCH 0/1] KVM: x86: using the fpu in interrupt context with a
 guest's xcr0

On Tue, Mar 15, 2016 at 8:48 PM, Andy Lutomirski <luto@...capital.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.