Re: [PATCH 2/2] KVM: arm64: Skip the cache flush when coalescing tables into a block

[wangyanan (Y)]

On 2021/3/9 0:34, Will Deacon wrote:
> On Mon, Jan 25, 2021 at 10:10:44PM +0800, Yanan Wang wrote:
> > After dirty-logging is stopped for a VM configured with huge mappings,
> > KVM will recover the table mappings back to block mappings. As we only
> > replace the existing page tables with a block entry and the cacheability
> > has not been changed, the cache maintenance opreations can be skipped.
> >
> > Signed-off-by: Yanan Wang <wangyanan55@xxxxxxxxxx>
> > ---
> >   arch/arm64/kvm/mmu.c | 12 +++++++++---
> >   1 file changed, 9 insertions(+), 3 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > index 8e8549ea1d70..37b427dcbc4f 100644
> > --- a/arch/arm64/kvm/mmu.c
> > +++ b/arch/arm64/kvm/mmu.c
> > @@ -744,7 +744,7 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> >   {
> >   	int ret = 0;
> >   	bool write_fault, writable, force_pte = false;
> > -	bool exec_fault;
> > +	bool exec_fault, adjust_hugepage;
> >   	bool device = false;
> >   	unsigned long mmu_seq;
> >   	struct kvm *kvm = vcpu->kvm;
> > @@ -872,12 +872,18 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> >   		mark_page_dirty(kvm, gfn);
> >   	}
> >   
> > -	if (fault_status != FSC_PERM && !device)
> > +	/*
> > +	 * There is no necessity to perform cache maintenance operations if we
> > +	 * will only replace the existing table mappings with a block mapping.
> > +	 */
> > +	adjust_hugepage = fault_granule < vma_pagesize ? true : false;
> nit: you don't need the '? true : false' part
>
> That said, your previous patch checks for 'fault_granule > vma_pagesize',
> so I'm not sure the local variable helps all that much here because it
> obscures the size checks in my opinion. It would be more straight-forward
> if we could structure the logic as:
>
>
> 	if (fault_granule < vma_pagesize) {
>
> 	} else if (fault_granule > vma_page_size) {
>
> 	} else {
>
> 	}
>
> With some comments describing what we can infer about the memcache and cache
> maintenance requirements for each case.
Thanks for your suggestion here, Will.
But I have resent another newer series [1] (KVM: arm64: Improve 
efficiency of stage2 page table)
recently, which has the same theme but different solutions that I think 
are better.
[1] 
https://lore.kernel.org/lkml/20210208112250.163568-1-wangyanan55@xxxxxxxxxx/

Could you please comment on that series ?  I think it can be found in 
your inbox :).

Thanks,

Yanan
> Will
> .