From mboxrd@z Thu Jan  1 00:00:00 1970
From: Andi Kleen <andi@firstfloor.org>
Subject: Re: [RFC PATCH 3/3] KVM: MMU: Optimize guest page table walk
Date: Fri, 29 Apr 2011 18:05:11 +0200
Message-ID: <20110429160511.GC3985@one.firstfloor.org>
References: <20110419033220.e527bcae.takuya.yoshikawa@gmail.com> <20110419033814.3cc7ab5e.takuya.yoshikawa@gmail.com> <4DAEA123.3020403@redhat.com> <m2liyt3hcn.fsf@firstfloor.org> <20110429143808.29c51c6a.takuya.yoshikawa@gmail.com> <20110429065956.GA3985@one.firstfloor.org> <20110429225148.49e6e700.takuya.yoshikawa@gmail.com>
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Cc: Andi Kleen <andi@firstfloor.org>, Avi Kivity <avi@redhat.com>,
	mtosatti@redhat.com, kvm@vger.kernel.org,
	yoshikawa.takuya@oss.ntt.co.jp
To: Takuya Yoshikawa <takuya.yoshikawa@gmail.com>
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> Do you think the following case would not differ so much
> from (1' 2') ?
> 
> walk_addr_generic()              ---1''
>   copy_from_user()               ---2''

Yes it should be the same and is cleaner.

If you do a make .../foo.i and look at the code coming out of the
preprocessor you'll see it expands to a 

	if (!__builtin_constant_p(size))
                return copy_user_generic(dst, (__force void *)src, size);
        switch (size) {
        case 1:__get_user_asm(*(u8 *)dst, (u8 __user *)src,
                              ret, "b", "b", "=q", 1);
                return ret;
	case 2: ..
	case 4: ..
	case 8: ..
	case 10: ..
	case 16: ..
	}

Ok it looks like the 32bit kernel only handles 1/2/4. Maybe that
was the problem if you ran on 32bit.

-Andi