From mboxrd@z Thu Jan 1 00:00:00 1970 From: Alexander Graf Date: Tue, 11 May 2010 16:00:42 +0000 Subject: Re: [RFC][PATCH 7/12 not tested yet] PPC: introduce __set_bit() like Message-Id: <4BE97F2A.5080900@suse.de> List-Id: References: <20100504215645.6448af8f.takuya.yoshikawa@gmail.com> <20100504220418.083929bc.takuya.yoshikawa@gmail.com> In-Reply-To: <20100504220418.083929bc.takuya.yoshikawa@gmail.com> MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Takuya Yoshikawa Cc: avi@redhat.com, mtosatti@redhat.com, yoshikawa.takuya@oss.ntt.co.jp, fernando@oss.ntt.co.jp, kvm@vger.kernel.org, kvm-ppc@vger.kernel.org, kvm-ia64@vger.kernel.org, tglx@linutronix.de, mingo@redhat.com, hpa@zytor.com, x86@kernel.org, benh@kernel.crashing.org, paulus@samba.org, linuxppc-dev@ozlabs.org, arnd@arndb.de, linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org Takuya Yoshikawa wrote: > During the work of KVM's dirty page logging optimization, we encountered > the need of manipulating bitmaps in user space efficiantly. To achive this, > we introduce a uaccess function for setting a bit in user space following > Avi's suggestion. > > KVM is now using dirty bitmaps for live-migration and VGA. Although we need > to update them from kernel side, copying them every time for updating the > dirty log is a big bottleneck. Especially, we tested that zero-copy bitmap > manipulation improves responses of GUI manipulations a lot. > > We also found one similar need in drivers/vhost/vhost.c in which the author > implemented set_bit_to_user() locally using inefficient functions: see TODO > at the top of that. > > Probably, this kind of need would be common for virtualization area. > > So we introduce a function set_bit_user_non_atomic(). > > Signed-off-by: Takuya Yoshikawa > Signed-off-by: Fernando Luis Vazquez Cao > CC: Alexander Graf > CC: Benjamin Herrenschmidt > CC: Paul Mackerras > --- > arch/powerpc/include/asm/uaccess.h | 19 +++++++++++++++++++ > 1 files changed, 19 insertions(+), 0 deletions(-) > > diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h > index 3a01ce8..f878326 100644 > --- a/arch/powerpc/include/asm/uaccess.h > +++ b/arch/powerpc/include/asm/uaccess.h > @@ -321,6 +321,25 @@ do { \ > __gu_err; \ > }) > > +static inline int set_bit_user_non_atomic(int nr, void __user *addr) > +{ > + u8 __user *p; > + u8 val; > + > + p = (u8 __user *)((unsigned long)addr + nr / BITS_PER_BYTE); > Does C do the + or the / first? Either way, I'd like to see brackets here :) Alex