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List:       kvm-ppc
Subject:    RE: [PATCH 5 of 5] kvm: powerpc: Map guest userspace with TID=0 mappings
From:       "Liu Yu" <Yu.Liu () freescale ! com>
Date:       2008-07-29 10:56:38
Message-ID: E20ABADA5792574791053954EC20B59509B69D () zch01exm26 ! fsl ! freescale ! net
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=-

> -----Original Message-----
> From: kvm-ppc-owner@vger.kernel.org 
> [mailto:kvm-ppc-owner@vger.kernel.org] On Behalf Of Liu Yu
> Sent: Tuesday, July 29, 2008 3:48 PM
> To: Christian Ehrhardt
> Cc: Hollis Blanchard; avi@qumranet.com; 
> kvm-ppc@vger.kernel.org; kvm@vger.kernel.org
> Subject: RE: [PATCH 5 of 5] kvm: powerpc: Map guest userspace 
> with TID=0 mappings
> 
> 
> > -----Original Message-----
> > From: Christian Ehrhardt [mailto:ehrhardt@linux.vnet.ibm.com]
> > Sent: Tuesday, July 29, 2008 3:03 PM
> > To: Liu Yu
> > Cc: Hollis Blanchard; avi@qumranet.com; kvm-ppc@vger.kernel.org; 
> > kvm@vger.kernel.org
> > Subject: Re: [PATCH 5 of 5] kvm: powerpc: Map guest userspace with 
> > TID=0 mappings
> > 
> > On Monday 28 July 2008 12:33:41 Liu Yu wrote:
> > > I have a question that I could not think through.
> > > While multiple qemu/kvm processes are running at the same
> > time, how to
> > > prevent one guest from using others' TLB? For all the
> > guests have the
> > > same TID=0 for userspace and TID=1 for kernel.
> > [...]
> > 
> > Hi Yu Liu, thats a good question.
> > Afaik thats solved by the fact that the shadow tlb which is 
> used when 
> > entering guest context is per vcpu. Therefor a guest has 
> always it's 
> > own shadow tlb active and no mappings to the content of 
> other guests.
> 
> Yes, shadow tlb is per vcpu.
> But in the patch 4/5, before entering guest context, not all 
> shadow tlb will be written back.
> So if (guest A -> guest B) happen, after entering guest B, is 
> there any possibility that A's tlb is still existing in hardware?

I see.
'kvm_arch_vcpu_load' and 'kvm_arch_vcpu_put' will handle this in preempt hooker.

> 
> 
> > 
> > This patch just allows us that a single guest userspace process 
> > accessing the kernel 20 times (and changing privilege level 
> 20 times 
> > by doing so) can run without tlb flushes.
> > Guest-userspace context switch (pid is changing) -> tlb flush; and 
> > guest switches (guest A -> guest B) -> other shadow tlb 
> active; should 
> > still be working fine.
> > 
> > > >
> > > > The net is that we don't need to flush the TLB on privilege 
> > > > switches, but we do on guest context switches (which 
> are far more 
> > > > infrequent). Guest boot time performance
> > improvement: about 30%.
> > > >
> > 
> > --
> > 
> > Grüsse / regards,
> > Christian Ehrhardt
> > IBM Linux Technology Center, Open Virtualization
> > 
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