Re: [PATCH v6 3/7] KVM: MMU: fast invalidate all pages

[Gleb Natapov]

On Wed, May 22, 2013 at 11:25:13PM +0800, Xiao Guangrong wrote:
> On 05/22/2013 09:17 PM, Gleb Natapov wrote:
> > On Wed, May 22, 2013 at 05:41:10PM +0800, Xiao Guangrong wrote:
> >> On 05/22/2013 04:54 PM, Gleb Natapov wrote:
> >>> On Wed, May 22, 2013 at 04:46:04PM +0800, Xiao Guangrong wrote:
> >>>> On 05/22/2013 02:34 PM, Gleb Natapov wrote:
> >>>>> On Tue, May 21, 2013 at 10:33:30PM -0300, Marcelo Tosatti wrote:
> >>>>>> On Tue, May 21, 2013 at 11:39:03AM +0300, Gleb Natapov wrote:
> >>>>>>>> Any pages with stale information will be zapped by kvm_mmu_zap_all().
> >>>>>>>> When that happens, page faults will take place which will automatically 
> >>>>>>>> use the new generation number.
> >>>>>>>>
> >>>>>>>> So still not clear why is this necessary.
> >>>>>>>>
> >>>>>>> This is not, strictly speaking, necessary, but it is the sane thing to do.
> >>>>>>> You cannot update page's generation number to prevent it from been
> >>>>>>> destroyed since after kvm_mmu_zap_all() completes stale ptes in the
> >>>>>>> shadow page may point to now deleted memslot. So why build shadow page
> >>>>>>> table with a page that is in a process of been destroyed?
> >>>>>>
> >>>>>> OK, can this be introduced separately, in a later patch, with separate
> >>>>>> justification, then?
> >>>>>>
> >>>>>> Xiao please have the first patches of the patchset focus on the problem
> >>>>>> at hand: fix long mmu_lock hold times.
> >>>>>>
> >>>>>>> Not sure what you mean again. We flush TLB once before entering this function.
> >>>>>>> kvm_reload_remote_mmus() does this for us, no?
> >>>>>>
> >>>>>> kvm_reload_remote_mmus() is used as an optimization, its separate from the
> >>>>>> problem solution.
> >>>>>>
> >>>>>>>>
> >>>>>>>> What was suggested was... go to phrase which starts with "The only purpose
> >>>>>>>> of the generation number should be to".
> >>>>>>>>
> >>>>>>>> The comment quoted here does not match that description.
> >>>>>>>>
> >>>>>>> The comment describes what code does and in this it is correct.
> >>>>>>>
> >>>>>>> You propose to not reload roots right away and do it only when root sp
> >>>>>>> is encountered, right? So my question is what's the point? There are,
> >>>>>>> obviously, root sps with invalid generation number at this point, so
> >>>>>>> reload will happen regardless in kvm_mmu_prepare_zap_page(). So why not
> >>>>>>> do it here right away and avoid it in kvm_mmu_prepare_zap_page() for
> >>>>>>> invalid and obsolete sps as I proposed in one of my email?
> >>>>>>
> >>>>>> Sure. But Xiao please introduce that TLB collapsing optimization as a
> >>>>>> later patch, so we can reason about it in a more organized fashion.
> >>>>>
> >>>>> So, if I understand correctly, you are asking to move is_obsolete_sp()
> >>>>> check from kvm_mmu_get_page() and kvm_reload_remote_mmus() from
> >>>>> kvm_mmu_invalidate_all_pages() to a separate patch. Fine by me, but if
> >>>>> we drop kvm_reload_remote_mmus() from kvm_mmu_invalidate_all_pages() the
> >>>>> call to kvm_mmu_invalidate_all_pages() in emulator_fix_hypercall() will
> >>>>> become nop. But I question the need to zap all shadow pages tables there
> >>>>> in the first place, why kvm_flush_remote_tlbs() is not enough?
> >>>>
> >>>> I do not know too... I even do no know why kvm_flush_remote_tlbs
> >>>> is needed. :(
> >>> We changed the content of an executable page, we need to flush instruction
> >>> cache of all vcpus to not use stale data, so my suggestion to call
> >>
> >> I thought the reason is about icache too but icache is automatically
> >> flushed on x86, we only need to invalidate the prefetched instructions by
> >> executing a serializing operation.
> >>
> >> See the SDM in the chapter of
> >> "8.1.3 Handling Self- and Cross-Modifying Code"
> >>
> > Right, so we do cross-modifying code here and we need to make sure no
> > vcpu is running in a guest mode while this happens, but
> > kvm_mmu_zap_all() does not provide this guaranty since vcpus will
> > continue running after reloading roots!
> 
> May be we can introduce a function to atomic write gpa, then the guest
> either 1) see the old value, in that case, it can be intercepted or
> 2) see the the new value in that case, it can continue to execute.
> 
SDM says atomic write is not enough. All vcpu should be guarantied to
not execute code in the vicinity of modified code. This is easy to
achieve though:

vcpu0:                            
lock(x);
make_all_cpus_request(EXIT);
unlock(x);

vcpuX:
if (kvm_check_request(EXIT)) { 
    lock(x);
    unlock(x);
}

> >>> kvm_flush_remote_tlbs() is obviously incorrect since this flushes tlb,
> >>> not instruction cache, but why kvm_reload_remote_mmus() would flush
> >>> instruction cache?
> >>
> >> kvm_reload_remote_mmus do not have any help i think.
> >>
> >> I find that this change is introduced by commit: 7aa81cc0
> >> and I have added Anthony in the CC.
> >>
> >> I also find some discussions related to calling
> >> kvm_reload_remote_mmus():
> >>
> >>>
> >>> But if the instruction is architecture dependent, and you run on the
> >>> wrong architecture, now you have to patch many locations at fault time,
> >>> introducing some nasty runtime code / data cache overlap performance
> >>> problems.  Granted, they go away eventually.
> >>>
> >>
> >> We're addressing that by blowing away the shadow cache and holding the
> >> big kvm lock to ensure SMP safety.  Not a great thing to do from a
> >> performance perspective but the whole point of patching is that the cost
> >> is amortized.
> >>
> >> (http://kerneltrap.org/mailarchive/linux-kernel/2007/9/14/260288)
> >>
> >> But i can not understand...
> > Back then kvm->lock protected memslot access so code like:
> > 
> > mutex_lock(&vcpu->kvm->lock);
> > kvm_mmu_zap_all(vcpu->kvm);
> > mutex_unlock(&vcpu->kvm->lock);
> > 
> > which is what 7aa81cc0 does was enough to guaranty that no vcpu will
> > run while code is patched. 
> 
> So, at that time, kvm->lock is also held when #PF is being fixed?
> 
It was, and also during kvm_mmu_load() which is called during vcpu entry
after roots are zapped.

> > This is no longer the case and
> > mutex_lock(&vcpu->kvm->lock); is gone from that code path long time ago,
> > so now kvm_mmu_zap_all() there is useless and the code is incorrect.
> > 
> > Lets drop kvm_mmu_zap_all() there (in separate patch) and fix the
> > patching properly later.
> 
> Will do.
> 

--
			Gleb.
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