Re: [PATCH kernel v2] KVM: PPC: Allocate guest TCEs on demand too

[Alexey Kardashevskiy <aik@ozlabs.ru>]

On 01/03/2019 12:38, Alexey Kardashevskiy wrote:
> We already allocate hardware TCE tables in multiple levels and skip
> intermediate levels when we can, now it is a turn of the KVM TCE tables.
> Thankfully these are allocated already in 2 levels.
> 
> This moves the table's last level allocation from the creating helper to
> kvmppc_tce_put() and kvm_spapr_tce_fault().
> 
> This adds kvmppc_rm_ioba_validate() to do an additional test if
> the consequent kvmppc_tce_put() needs a page which has not been allocated;
> if this is the case, we bail out to virtual mode handlers.
> 
> Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
> ---
> Changes:
> v2:
> * added kvm mutex around alloc_page to prevent races; in both place we
> test the pointer, if NULL, then take a lock and check again so on a fast
> path we do not take a lock at all
> 
> 
> ---
> For NVLink2 passthrough guests with 128TiB DMA windows and very fragmented
> system RAM the difference is gigabytes of RAM.
> ---
>  arch/powerpc/kvm/book3s_64_vio.c    | 29 ++++++------
>  arch/powerpc/kvm/book3s_64_vio_hv.c | 69 ++++++++++++++++++++++++++---
>  2 files changed, 79 insertions(+), 19 deletions(-)
> 
> diff --git a/arch/powerpc/kvm/book3s_64_vio.c b/arch/powerpc/kvm/book3s_64_vio.c
> index f02b049..7eed8c9 100644
> --- a/arch/powerpc/kvm/book3s_64_vio.c
> +++ b/arch/powerpc/kvm/book3s_64_vio.c
> @@ -228,7 +228,8 @@ static void release_spapr_tce_table(struct rcu_head *head)
>  	unsigned long i, npages = kvmppc_tce_pages(stt->size);
>  
>  	for (i = 0; i < npages; i++)
> -		__free_page(stt->pages[i]);
> +		if (stt->pages[i])
> +			__free_page(stt->pages[i]);
>  
>  	kfree(stt);
>  }
> @@ -242,6 +243,20 @@ static vm_fault_t kvm_spapr_tce_fault(struct vm_fault *vmf)
>  		return VM_FAULT_SIGBUS;
>  
>  	page = stt->pages[vmf->pgoff];
> +	if (!page) {
> +		mutex_lock(&stt->kvm->lock);
> +		page = stt->pages[vmf->pgoff];
> +		if (!page) {
> +			page  = alloc_page(GFP_KERNEL | __GFP_ZERO);
> +			if (!page) {
> +				mutex_unlock(&stt->kvm->lock);
> +				return VM_FAULT_OOM;
> +			}
> +			stt->pages[vmf->pgoff] = page;
> +		}
> +		mutex_unlock(&stt->kvm->lock);
> +	}
> +
>  	get_page(page);
>  	vmf->page = page;
>  	return 0;
> @@ -296,7 +311,6 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm,
>  	struct kvmppc_spapr_tce_table *siter;
>  	unsigned long npages, size = args->size;
>  	int ret = -ENOMEM;
> -	int i;
>  
>  	if (!args->size || args->page_shift < 12 || args->page_shift > 34 ||
>  		(args->offset + args->size > (ULLONG_MAX >> args->page_shift)))
> @@ -320,12 +334,6 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm,
>  	stt->kvm = kvm;
>  	INIT_LIST_HEAD_RCU(&stt->iommu_tables);
>  
> -	for (i = 0; i < npages; i++) {
> -		stt->pages[i] = alloc_page(GFP_KERNEL | __GFP_ZERO);
> -		if (!stt->pages[i])
> -			goto fail;
> -	}
> -
>  	mutex_lock(&kvm->lock);
>  
>  	/* Check this LIOBN hasn't been previously allocated */
> @@ -352,11 +360,6 @@ long kvm_vm_ioctl_create_spapr_tce(struct kvm *kvm,
>  	if (ret >= 0)
>  		return ret;
>  
> - fail:
> -	for (i = 0; i < npages; i++)
> -		if (stt->pages[i])
> -			__free_page(stt->pages[i]);
> -
>  	kfree(stt);
>   fail_acct:
>  	kvmppc_account_memlimit(kvmppc_stt_pages(npages), false);
> diff --git a/arch/powerpc/kvm/book3s_64_vio_hv.c b/arch/powerpc/kvm/book3s_64_vio_hv.c
> index 2206bc7..a0912d5 100644
> --- a/arch/powerpc/kvm/book3s_64_vio_hv.c
> +++ b/arch/powerpc/kvm/book3s_64_vio_hv.c
> @@ -158,23 +158,76 @@ static u64 *kvmppc_page_address(struct page *page)
>  	return (u64 *) page_address(page);
>  }
>  
> +/*
> + * TCEs pages are allocated in kvmppc_tce_put() which won't be able to do so
> + * in real mode.
> + * Check if kvmppc_tce_put() can succeed in real mode, i.e. a TCEs page is
> + * allocated or not required (when clearing a tce entry).
> + */
> +static long kvmppc_rm_ioba_validate(struct kvmppc_spapr_tce_table *stt,
> +		unsigned long ioba, unsigned long npages, bool clearing)
> +{
> +	unsigned long i, sttpage, sttpages;
> +	unsigned long ret = kvmppc_ioba_validate(stt, ioba, npages);
> +
> +	if (ret)
> +		return ret;
> +	/*
> +	 * clearing==true says kvmppc_tce_put won't be allocating pages
> +	 * for empty tces.
> +	 */
> +	if (clearing)
> +		return H_SUCCESS;
> +
> +	sttpage = ((ioba >> stt->page_shift) - stt->offset) / TCES_PER_PAGE;
> +	sttpages = (npages + TCES_PER_PAGE - 1) / TCES_PER_PAGE;


This is wrong, v3 is coming.


-- 
Alexey