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

["wangyanan (Y)" <wangyanan55@huawei.com>]

On 2021/3/9 16:43, Marc Zyngier wrote:
> On Tue, 09 Mar 2021 08:34:43 +0000,
> "wangyanan (Y)" <wangyanan55@huawei.com> wrote:
>>
>> 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@huawei.com>
>>>> ---
>>>>    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@huawei.com/
>>
>> Could you please comment on that series ?  I think it can be found in
>> your inbox :).
> There were already a bunch of comments on that series, and I stopped
> at the point where the cache maintenance was broken. Please respin
> that series if you want further feedback on it.
Ok, I will send a new version later.
>
> In the future, if you deprecate a series (which is completely
> understandable), please leave a note on the list with a pointer to the
> new series so that people don't waste time reviewing an obsolete
> series. Or post the new series with a new version number so that it is
> obvious that the original series has been superseded.
I apologize for this, I will be more careful in the future.

Thanks,

Yanan
> Thanks,
>
> 	M.
>
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