linux-kernel - Re: [PATCH v2 6/7] kvm,x86: RCU based table free

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Date:	Tue, 5 Jun 2012 12:58:32 +0100
From:	Stefano Stabellini <stefano.stabellini@...citrix.com>
To:	Nikunj A Dadhania <nikunj@...ux.vnet.ibm.com>
CC:	Stefano Stabellini <Stefano.Stabellini@...citrix.com>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"mingo@...e.hu" <mingo@...e.hu>,
	"mtosatti@...hat.com" <mtosatti@...hat.com>,
	"avi@...hat.com" <avi@...hat.com>,
	"raghukt@...ux.vnet.ibm.com" <raghukt@...ux.vnet.ibm.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"jeremy@...p.org" <jeremy@...p.org>,
	"vatsa@...ux.vnet.ibm.com" <vatsa@...ux.vnet.ibm.com>,
	"hpa@...or.com" <hpa@...or.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH v2 6/7] kvm,x86: RCU based table free

On Tue, 5 Jun 2012, Nikunj A Dadhania wrote:
> On Tue, 5 Jun 2012 11:48:02 +0100, Stefano Stabellini <stefano.stabellini@...citrix.com> wrote:
> > 
> > I am also interested in introducing HAVE_RCU_TABLE_FREE on x86 for Xen.
> > Maybe we can pull our efforts together :-)
> > 
> > Giving a look at this patch, it doesn't look like it is introducing
> > CONFIG_HAVE_RCU_TABLE_FREE anywhere under arch/x86.
> > How is the user supposed to set it?
> >
> I am doing that in the next patch only for KVM-ParavirtTLB flush, as
> there is a bug in this implementation that patch [7/7] fixes.
> 
> Refer following thread for details:
> http://mid.gmane.org/1337254086.4281.26.camel@twins
> http://mid.gmane.org/1337273959.4281.62.camel@twins

Thanks, somehow I missed the 7/7 patch.

>From the Xen POV, your patch is fine because we'll just select
PARAVIRT_TLB_FLUSH on CONFIG_XEN (see appended patch for completeness).

The main difference between the two approaches is that a kernel with
PARAVIRT_TLB_FLUSH and/or CONFIG_XEN enabled is going to have
HAVE_RCU_TABLE_FREE even when running on native.

Are you proposing this series for 3.5?
If not (because it depends on ticketlocks and KVM Paravirt Spinlock
patches), could you extract patch 6/7 and 7/7 and send them out
separately?
I am saying this because Xen needs the HAVE_RCU_TABLE_FREE fix even if
pv ticketlock are not accepted. This is an outstanding bug for us
unfortunately.

---

xen: select PARAVIRT_TLB_FLUSH if SMP

At the moment get_user_pages_fast is unsafe on Xen, because it relies on
the fact that flush_tlb_others sends an IPI to flush the tlb but
xen_flush_tlb_others doesn't send any IPIs and always returns
succesfully and immediately.

Select PARAVIRT_TLB_FLUSH, that enables an RCU lock to protect this kind
of software pagetable walks (also see HAVE_RCU_TABLE_FREE and
include/asm-generic/tlb.h).

Signed-off-by: Stefano Stabellini <stefano.stabellini@...citrix.com>

diff --git a/arch/x86/xen/Kconfig b/arch/x86/xen/Kconfig
index fdce49c..18c9876 100644
--- a/arch/x86/xen/Kconfig
+++ b/arch/x86/xen/Kconfig
@@ -6,6 +6,7 @@ config XEN
 	bool "Xen guest support"
 	select PARAVIRT
 	select PARAVIRT_CLOCK
+	select PARAVIRT_TLB_FLUSH if SMP
 	depends on X86_64 || (X86_32 && X86_PAE && !X86_VISWS)
 	depends on X86_CMPXCHG && X86_TSC
 	help
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