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

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Message-ID: <505AE281.1030603@redhat.com>
Date:	Thu, 20 Sep 2012 12:31:45 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Suresh Siddha <suresh.b.siddha@...el.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 09/19/2012 08:25 PM, Suresh Siddha wrote:
> 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.

kernel_fpu_begin() can allocate it.  It means changing the APIs, but
changing the behaviour to be preemptible is a bigger change anyway.


-- 
error compiling committee.c: too many arguments to function
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