linux-kernel - Re: [PATCH 1/1] KVM: don't allow irq_fpu_usable when the VCPU's XCR0 is loaded

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Message-ID: <CALCETrVc-GaqWC+MZm08YOUCMumzQ-TJK4h296LMtpQPdPS=0Q@mail.gmail.com>
Date:	Fri, 11 Mar 2016 13:14:20 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	David Matlack <dmatlack@...gle.com>
Cc:	"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>,
	Andrew Lutomirski <luto@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>, digitaleric@...gle.com
Subject: Re: [PATCH 1/1] KVM: don't allow irq_fpu_usable when the VCPU's XCR0
 is loaded

On Fri, Mar 11, 2016 at 12:47 PM, David Matlack <dmatlack@...gle.com> wrote:
> From: Eric Northup <digitaleric@...gle.com>
>
> Add a percpu boolean, tracking whether a KVM vCPU is running on the
> host CPU.  KVM will set and clear it as it loads/unloads guest XCR0.
> (Note that the rest of the guest FPU load/restore is safe, because
> kvm_load_guest_fpu and kvm_put_guest_fpu call __kernel_fpu_begin()
> and __kernel_fpu_end(), respectively.)  irq_fpu_usable() will then
> also check for this percpu boolean.

Is this better than just always keeping the host's XCR0 loaded outside
if the KVM interrupts-disabled region?

--Andy