Subject: Re: [PATCH v5 15/19] KVM: Provide common implementation for generic dirty log functions

From: Sean Christopherson <[email-masked]>
Date: Thu, 6 Feb 2020 13:21:20 -0800

On Thu, Feb 06, 2020 at 03:02:00PM -0500, Peter Xu wrote:
> On Tue, Jan 21, 2020 at 02:31:53PM -0800, Sean Christopherson wrote:
> 
> [...]
> 
> > -int kvm_vm_ioctl_clear_dirty_log(struct kvm *kvm, struct kvm_clear_dirty_log *log)
> > +void kvm_arch_dirty_log_tlb_flush(struct kvm *kvm,
> > +				  struct kvm_memory_slot *memslot)
> 
> If it's to flush TLB for a memslot, shall we remove the "dirty_log" in
> the name of the function, because it has nothing to do with dirty
> logging any more?  And...

I kept the "dirty_log" to allow arch code to implement logic specific to a
TLB flush during dirty logging, e.g. x86's lockdep assert on slots_lock.
And similar to the issue with MIPS below, to deter usage of the hook for
anything else, i.e. to nudge people to using kvm_flush_remote_tlbs()
directly.

> >  {
> > -	struct kvm_memslots *slots;
> > -	struct kvm_memory_slot *memslot;
> > -	bool flush = false;
> > -	int r;
> > -
> > -	mutex_lock(&kvm->slots_lock);
> > -
> > -	r = kvm_clear_dirty_log_protect(kvm, log, &flush);
> > -
> > -	if (flush) {
> > -		slots = kvm_memslots(kvm);
> > -		memslot = id_to_memslot(slots, log->slot);
> > -
> > -		/* Let implementation handle TLB/GVA invalidation */
> > -		kvm_mips_callbacks->flush_shadow_memslot(kvm, memslot);
> > -	}
> > -
> > -	mutex_unlock(&kvm->slots_lock);
> > -	return r;
> > +	/* Let implementation handle TLB/GVA invalidation */
> > +	kvm_mips_callbacks->flush_shadow_memslot(kvm, memslot);
> 
> ... This may not directly related to the current patch, but I'm
> confused on why MIPS cannot use kvm_flush_remote_tlbs() to flush TLBs.
> I know nothing about MIPS code, but IIUC here flush_shadow_memslot()
> is a heavier operation that will also invalidate the shadow pages.
> Seems to be an overkill here when we only changed write permission of
> the PTEs?  I tried to check the first occurance (2a31b9db15353) but I
> didn't find out any clue of it so far.
> 
> But that matters to this patch because if MIPS can use
> kvm_flush_remote_tlbs(), then we probably don't need this
> arch-specific hook any more and we can directly call
> kvm_flush_remote_tlbs() after sync dirty log when flush==true.

Ya, the asid_flush_mask in kvm_vz_flush_shadow_all() is the only thing
that prevents calling kvm_flush_remote_tlbs() directly, but I have no
clue as to the important of that code.

> >  }
> >  
> >  long kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
> > diff --git a/arch/powerpc/kvm/book3s.c b/arch/powerpc/kvm/book3s.c
> > index 97ce6c4f7b48..0adaf4791a6d 100644
> > --- a/arch/powerpc/kvm/book3s.c
> > +++ b/arch/powerpc/kvm/book3s.c
> > @@ -799,6 +799,11 @@ int kvmppc_core_check_requests(struct kvm_vcpu *vcpu)
> >  	return vcpu->kvm->arch.kvm_ops->check_requests(vcpu);
> >  }
> >  
> > +void kvm_arch_sync_dirty_log(struct kvm *kvm, struct kvm_memory_slot *memslot)
> 
> Since at it, maybe we can start to use __weak attribute for new hooks
> especially when it's empty for most archs?
> 
> E.g., define:
> 
> void __weak kvm_arch_sync_dirty_log(...) {}
> 
> In the common code, then only define it again in arch that has
> non-empty implementation of this method?

I defer to Paolo, I'm indifferent at this stage.

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