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

[Peter Xu]

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...

>  {
> -	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.

>  }
>  
>  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?

-- 
Peter Xu