Re: [PATCH v5 6/6] KVM: arm64: Distinguish cases of memcache allocations completely

[wangyanan (Y)]

Hi Quentin,

On 2021/6/2 19:07, Quentin Perret wrote:
> On Thursday 15 Apr 2021 at 19:50:32 (+0800), Yanan Wang wrote:
> > With a guest translation fault, the memcache pages are not needed if KVM
> > is only about to install a new leaf entry into the existing page table.
> > And with a guest permission fault, the memcache pages are also not needed
> > for a write_fault in dirty-logging time if KVM is only about to update
> > the existing leaf entry instead of collapsing a block entry into a table.
> >
> > By comparing fault_granule and vma_pagesize, cases that require allocations
> > from memcache and cases that don't can be distinguished completely.
> >
> > Signed-off-by: Yanan Wang <wangyanan55@xxxxxxxxxx>
> > ---
> >   arch/arm64/kvm/mmu.c | 25 ++++++++++++-------------
> >   1 file changed, 12 insertions(+), 13 deletions(-)
> >
> > diff --git a/arch/arm64/kvm/mmu.c b/arch/arm64/kvm/mmu.c
> > index aa536392b308..9e35aa5d29f2 100644
> > --- a/arch/arm64/kvm/mmu.c
> > +++ b/arch/arm64/kvm/mmu.c
> > @@ -895,19 +895,6 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> >   	gfn = fault_ipa >> PAGE_SHIFT;
> >   	mmap_read_unlock(current->mm);
> >   
> > -	/*
> > -	 * Permission faults just need to update the existing leaf entry,
> > -	 * and so normally don't require allocations from the memcache. The
> > -	 * only exception to this is when dirty logging is enabled at runtime
> > -	 * and a write fault needs to collapse a block entry into a table.
> > -	 */
> > -	if (fault_status != FSC_PERM || (logging_active && write_fault)) {
> > -		ret = kvm_mmu_topup_memory_cache(memcache,
> > -						 kvm_mmu_cache_min_pages(kvm));
> > -		if (ret)
> > -			return ret;
> > -	}
> > -
> >   	mmu_seq = vcpu->kvm->mmu_notifier_seq;
> >   	/*
> >   	 * Ensure the read of mmu_notifier_seq happens before we call
> > @@ -970,6 +957,18 @@ static int user_mem_abort(struct kvm_vcpu *vcpu, phys_addr_t fault_ipa,
> >   	else if (cpus_have_const_cap(ARM64_HAS_CACHE_DIC))
> >   		prot |= KVM_PGTABLE_PROT_X;
> >   
> > +	/*
> > +	 * Allocations from the memcache are required only when granule of the
> > +	 * lookup level where the guest fault happened exceeds vma_pagesize,
> > +	 * which means new page tables will be created in the fault handlers.
> > +	 */
> > +	if (fault_granule > vma_pagesize) {
> > +		ret = kvm_mmu_topup_memory_cache(memcache,
> > +						 kvm_mmu_cache_min_pages(kvm));
> > +		if (ret)
> > +			return ret;
> > +	}
> You're now doing the top-up in the kvm->mmu_lock critical section. Isn't
> this more or less what we try to avoid by using a memory cache?
Oh, right!

This patch intended to clean up the code and avoid the unnecessary top-ups,
but it's a bad idea to do the top-up when holding mmu_lock. I will rearrange
this part and keep it where it should be.

Thanks,
Yanan
> Thanks,
> Quentin
> .