linux-kernel - Re: [PATCH 3/6] x86, kvm: use kernel_fpu_begin/end() in kvm_load/put_guest

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Date:	Wed, 19 Sep 2012 10:25:33 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	hpa@...or.com, mingo@...nel.org, torvalds@...ux-foundation.org,
	andreas.herrmann3@....com, bp@...64.org, robert.richter@....com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/6] x86, kvm: use kernel_fpu_begin/end() in
 kvm_load/put_guest_fpu()

On Wed, 2012-09-19 at 20:22 +0300, Avi Kivity wrote:
> On 09/19/2012 08:18 PM, Suresh Siddha wrote:
> 
> > These routines (kvm_load/put_guest_fpu()) are already called with
> > preemption disabled but as you mentioned, we don't want the preemption
> > to be disabled completely between the kvm_load_guest_fpu() and
> > kvm_put_guest_fpu().
> > 
> > Also KVM already has the preempt notifier which is doing the
> > kvm_put_guest_fpu(), so something like the appended should address this.
> > I will test this shortly.
> > 
> 
> Note, we could also go in a different direction and make
> kernel_fpu_begin() use preempt notifiers and thus make its users
> preemptible.  But that's for a separate patchset.

yep, but we need the fpu buffer to save/restore the kernel fpu state.

KVM already has those buffers allocated in the guest cpu state and hence
it all works out ok. But yes, we can revisit this in future.

thanks,
suresh


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