From mboxrd@z Thu Jan 1 00:00:00 1970 From: Xiao Guangrong Subject: Re: [PATCH v6 6/9] KVM: MMU: fast path of handling guest page fault Date: Thu, 14 Jun 2012 11:00:14 +0800 Message-ID: <4FD953BE.6020200@linux.vnet.ibm.com> References: <4FC470C7.5040700@linux.vnet.ibm.com> <4FC471B8.3070204@linux.vnet.ibm.com> <20120613224002.GE19290@amt.cnet> Mime-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Cc: Avi Kivity , LKML , KVM To: Marcelo Tosatti Return-path: In-Reply-To: <20120613224002.GE19290@amt.cnet> Sender: linux-kernel-owner@vger.kernel.org List-Id: kvm.vger.kernel.org On 06/14/2012 06:40 AM, Marcelo Tosatti wrote: > On Tue, May 29, 2012 at 02:50:32PM +0800, Xiao Guangrong wrote: >> If the the present bit of page fault error code is set, it indicates >> the shadow page is populated on all levels, it means what we do is >> only modify the access bit which can be done out of mmu-lock >> >> Currently, in order to simplify the code, we only fix the page fault >> caused by write-protect on the fast path >> >> Signed-off-by: Xiao Guangrong >> --- >> arch/x86/kvm/mmu.c | 126 +++++++++++++++++++++++++++++++++++++++++++++++----- >> 1 files changed, 114 insertions(+), 12 deletions(-) >> >> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c >> index 150c5ad..d6101a8 100644 >> --- a/arch/x86/kvm/mmu.c >> +++ b/arch/x86/kvm/mmu.c >> @@ -445,6 +445,11 @@ static bool __check_direct_spte_mmio_pf(u64 spte) >> } >> #endif >> >> +static bool spte_can_be_writable(u64 spte) >> +{ >> + return !(~spte & (SPTE_HOST_WRITEABLE | SPTE_MMU_WRITEABLE)); >> +} >> + > > spte_is_locklessly_modifiable(). Its easy to confuse > "spte_can_be_writable" with different things. > Yes. Will update it. >> static bool spte_has_volatile_bits(u64 spte) >> { >> if (!shadow_accessed_mask) >> @@ -454,7 +459,7 @@ static bool spte_has_volatile_bits(u64 spte) >> return false; >> >> if ((spte & shadow_accessed_mask) && >> - (!is_writable_pte(spte) || (spte & shadow_dirty_mask))) >> + (!spte_can_be_writable(spte) || (spte & shadow_dirty_mask))) >> return false; > > mmu_spte_update is handling several different cases. Please rewrite > it, add a comment on top of it (or spread comments on top of each > significant code line) with all cases it is handling (also recheck it > regarding new EPT accessed/dirty bits code). > Okay. > For one thing, if spte can be updated locklessly the update must be > atomic: > > if spte can be locklessly updated > read-and-modify must be atomic. Actually, i did it in the v5, Avi has some comments on that. Please see https://lkml.org/lkml/2012/5/24/55 What the reason we should locklessly update spte here? So far i know is for volatile bit lost and getting a stable is_writable_spte()? But this two cases can be avoided by using spte_can_be_writable(spte) instead of is_writable_pte(spte), right?